Much has been said already but I'd like to suggest to try and be a bit more careful in the formalization. Stars and underbars are not really used anywhere in the literature I've seen so far and I find them distracting. Also, minimization of the axioms is not always the royal way to clarity.
I'm going to give a detailed categorical definition of groupoids and then indicate how to relate it to the algebraic one you ask about and see that they are equivalent. I'll then briefly address your second question and towards the end I'll try and give some glimpses towards an answer of your broad question 3. I'm making my life a bit more complicated than necessary in that I don't introduce Hom-sets (or only implicitly) but that's because groupoids per se are much more interesting when considered in categories other than $\mathbf{Sets}$ and in order to be able to do so, the following definition is more adequate (view it as a preparation for my attempt at answering your question 3).
So here we go in full detail:
A groupoid $G$ consists of two sets: a set of objects $G_0$ and a set of arrows $G_1$ together with five structure maps $(s,t,u,i,c)$
- source and target $s,t: G_1 \to G_0$ (domain and codomain of arrows).
- unit $u: G_0 \to G_1$ (the unit inclusion mapping an object to its identity morphism)
- composition of arrows $c: G_2 \to G_1$, defined on the fibered product (pullback) $$G_{2} = G_1 {}_s{\times}_{t}\ G_1 = \{(h,g) \in G_1 \times G_1\,:\,s(h)=t(g)\}$$ and written $c(h,g) = hg$. (The set $G_2$ is the set of pairs of composable arrows).
- inversion of arrows $i: G_1 \to G_1$ written $i(g) = g^{-1}$
subject to the following conditions
- compatibility of source and target with composition: for all $(h,g) \in G_2$ we have $s(hg) = s(g)$ and $t(hg) = t(h)$.
- associativity: for all $k,h,g \in G_1$ such that $s(k) = t(h)$ and $s(h)=t(g)$ we have $(kh)g = k(hg)$ (note that this makes sense in view of 1).
- compatibility of unit with source, target and composition:
- For all $x \in G_0$ we have $su(x) = x = tu(x)$
- For all $g \in G_1$ we have $[ut(g)]g = g = g[us(g)]$
- compatibilities of the inverse: For all $g \in G_1$ we have $s(g^{-1}) = t(g)$ and $t(g^{-1}) = s(g)$ as well as $g^{-1}g = us(g)$ and $gg^{-1}=ut(g)$.
All this can be expressed in purely diagrammatic terms and leads to the notion of a groupoid inside a category (and makes sense as soon as there are enough pullbacks). Also when you flesh out the definition of a category on Wikipedia you see that the Hom-sets are simply $G(x,y) = \{g \in G_1:s(g) = x\text{ and }t(g) = y\}$ and that the structure maps above are implicitly built into the formalism.
I'm of the opinion that only this is a reasonable (category-theoretic) definition of groupoids and everything else is just cheating one's way around precision. When saying that one has a (small) category the usual formulation is exactly what is obtained when dropping $i$ and the two points 4. above.
So, when people are saying that "a groupoid is a small category in which every morphism is an isomorphism" then they mean first that they have a category, i.e., two sets $G_0$ and $G_1$ and four structure maps $(s,t,u,c)$ satisfying points 1., 2., and 3. above and 4. is what one ends up with when one thinks about what it really means for a(ll) morphism(s) to be invertible (isomorphisms). So much for the categorical part of your question 1.
Now I should relate this to the definition of algebraic groupoids. From the above it should be clear that given a groupoid in the above sense, $\ast = c$ is a partial operation satisfying associativity, that "inverses are always defined" and that the "identity" axiom is satisfied.
Now we need to check that we can go back from the algebraic definition to the category theoretic definition, so we need to identify the sets $G_1$ and $G_0$ and the structure maps. Here are the details (bear with me and let me drop the star):
- Put $G_1 = G$ and $G_0 = \{g \in G: g^{-1} = g\}$ (idea: objects are the same thing as identity morphisms and they are distinguished by being their own inverses) the unit $u:G_0 \to G_1$ is simply the inclusion and inversion $i: G_1 \to G_1$ is already given.
- Source is $s(g) = g^{-1}g$ and target is $t(g) = gg^{-1}$ these are well-defined because "inverses are always defined" and are maps $s,t: G_1 \to G_0$ because of your "easy and convenient property 2.".
- It remains to identify the domain of definition of multiplication with $G_2$ above (that's already in Arturo's answer). As we already spelled out what source and target are, we need to check that $hg$ is defined whenever $h^{-1}h = s(h) \stackrel{!}{=} t(g) = gg^{-1}$ and conversely.
If $h^{-1}h = gg^{-1}$ then $h = h(h^{-1}h)$ (take $a = b = h$ in the identity axiom) hence $h = h(h^{-1}h) = h(gg^{-1}) = (hg)g^{-1}$. But we can compose the last expression with $g$ from the right and get $(hg)g^{-1}g = hg$, hence $s(h) = t(g)$ implies that $hg$ is defined by the associativity axiom.
Conversely if $hg$ is defined then $hgg^{-1} = h$ hence composing this with $h^{-1}$ from the left we get $s(g) = gg^{-1} = h^{-1}h = t(h)$.
With all this preparation you should now have no trouble verifying the axioms 1. to 4. of the categorical definition.
Question 2 is rather easy to answer:
A morphism $\phi: G \to G'$ between groupoids is just a functor. In the spirit of the above definition let me spell out the details. I'm using primes to denote the structure maps of $G'$:
It consists of two maps $\phi_{1} : G_1 \to G_1^\prime$ and $\phi_{0}: G_{0} \to G_{0}^\prime$ satisfying the following compatibilities with the structure maps:
- source and target: $\phi_0 s = s'\phi_1$ and $\phi_0 t = t' \phi_1$
- unit: $\phi_1 u = u' \phi_0$
- composition: if $s(h) = t(g)$ then $\phi_1(hg) = \phi_1(h)\phi_1(g)$.
A more formal way to express the composition axiom 3. would be to say that $\phi_1$ induces a unique map $\phi_2 = \phi_1 {}_s{\times}_{t}\ \phi_1: G_2 \to G_{2}^{\prime}$ such that $(s',t') \circ \phi_2 = \phi_1 \circ (s,t)$ and the condition then becomes $c' \circ \phi_2 = \phi_1 \circ c$.
Compatibility with inverses is then automatic: $\phi_1 i = i'\phi_1$.
A subgroupoid is usually defined to be a groupoid $H$ together with a functor $\phi: H \to G$ such that $\phi_1$ and $\phi_0$ are injective (embeddings). This means that $\phi_1: H_1 \to \phi_1(H_1)$ is an isomorhpism onto its image and so is $\phi_0: H_0 \to \phi_{0}(H_0)$.
A homomorphism of algebraic groupoids is simply a map $\phi: G \to H$ that is compatible with composition: whenever $hg$ is defined then so is $\phi(h)\phi(g)$ and it is equal to $\phi(hg)$. The description of arrows, objects, source, target unit and composition then yield that a homomorphism of algebraic groupoids yields a functor and vice versa. In particular the categories of algebraic groupoids and homomorphisms and the category of groupoids-as-categories are equivalent.
Note: Starting with an algebraic groupoid and passing to the associated category and going back to the algebraic setting, one gets the algebraic groupoid back. On the other hand, starting with a categorical groupoid and passing to the associated algebraic groupoid and going back one gets an isomorphic categorical groupoid but not necessarily the same one because we didn't insist on $G_0$ being a subset of $G_1$ in the categorical definition. One could add that requirement and get an isomorphism of categories, but neither do I see a compelling reason to do so, nor is it usually done, as far as I can tell.
One should also mention that not only are there functors but there are also natural transformations, so, as the category of (small) categories is not only a category but a $2$-category, so is the category of groupoids. But entering more detail would mean to go too far in this already very long answer. Let me just mention that this allows one to consider groupoids up to equivalence of categories; there is the even weaker notion of Morita equivalence that is of considerable interest from a geometric point of view.
Here are three basic examples of groupoids:
- Groups.
- Equivalence relations.
- Group actions.
A group is a groupoid in which $G_{0}$ is reduced to a singleton. An equivalence relation is a groupoid for which there is at most one morphism between any two objects. Group actions don't have such a handy characterization but to every action of a group $G$ on a set $X$ one can associate the so-called action groupoid (or translation groupoid) $G \ltimes X$.
Many theorems on groupoids had earlier variants for groups, equivalence relations and action groupoids and were then generalized later on. This is one criticism that one often hears: there are few results that go in the other direction, at least not on a basic or immediate level — some people see groupoids as a way of formalizing analogies that were already self-evident before.
As I mentioned in a comment, groupoids on a set-theoretic level are already interesting and the book by Higgins, Categories and Groupoids is a nice introduction to that with e.g. a groupoid version Grushko's theorem and explaining the use in algebraic topology via the fundamental groupoid with the groupoid version of van Kampen as a highlight.
This leads me to my second recommendation: May's Concise course in algebraic topology shows the use of groupoids in a nice formulation of the theory of covering spaces, among other things like van Kampen, of course.
Since you are already aware of Ronald Brown's work, I'm not elaborating on that. However I must say that I think that characterizing Brown as "one of the founders of their study" as was done in one of the comments is a bit unfair considering the earlier and deep work e.g. by Ehresmann and Haefliger upon which Brown's first work on groupoids was built (I don't know Brown personally but from his homage to Ehresmann it seems that he is more than happy to acknowledge that). At some point I should also mention that groupoids were introduced by Heinrich Brandt in 1940.
Your third question
Question 3: (a too broad question, I know, but anything will be useful) Are there any alternate definitions of a groupoid? I'm aiming for one that is used most often throughout mathematics nowadays. How/in what fields are groupoids most often used? Algebraic topology? Are they given topological structure? What are some important theorems regarding groupoids?
is very broad so I'll give a broad answer, just dropping some key-words and giving some links. I should also add the Disclaimer that I'm far from being an expert on groupoids I just happen to have run into them many times already. I'm not aspiring to any sort of comprehensiveness and I'm just mentioning things that came to mind immediately. One further problem is that groupoids are mostly used at a fairly advanced level, so mentioning some scary words is almost unavoidable. Also, the interesting theorems on (or using) groupoids I'm aware of all seem to involve baggage way beyond the scope of this question and this answer.
Briefly: It seems fair to say that in every sufficiently algebraized field there are people considering and using groupoids.
- I'm not aware of significantly different definitions than the two you gave (but see e.g. this PlanetMath article on Brandt groupoids).
- The category-theoretic one is certainly by far more often used than the algebraic one.
- There are many versions of structured groupoids (smooth, topological, measure-theoretic and by means of algebro-geometric methods), as I mention below.
- I can't tell where groupoids are most often used.
A very important, popular and thus highly developed field of investigation is in differential geometry via Lie groupoids. Lie groupoids simultaneously encompass Lie groups and manifolds and on a more advanced level arise from foliations in guise of the holonomy groupoids. This is one of the reasons I spelled out the definition in such painstaking detail above: In order to define a Lie groupoid one needs to be a bit careful on what conditions to impose on $G_1$, $G_0$ and the structure maps. The most straightforward definition ($G_0$ and $G_1$ manifolds and all maps smooth) turns out not to be quite the right one and in order to spell the "good" definition out one needs to do some careful bookkeeping and there's no way around writing down all the structure maps.
Lie groupoids have associated Lie algebroids which are of considerable interest too, very much so in mathematical physics.
As a first step into this direction I recommend the surveys by Alan Weinstein, Groupoids: unifying internal and external symmetry and The geometry of momentum. For a more in-depth view I recommend the book by Weinstein-Canas da Silva, Geometric models of non-commutative algebras for a start. There are several surveys by Moerdijk and Mrčun that you can locate easily if you want to dig deeper.
In representation theory they arise e.g. in the Tannakian formalism (I couldn't find a good and digestible link on that).
In ergodic theory and representation theory of locally compact groups they are used in guise of Mackey's virtual groups, roughly a theory built on a measure-theoretic version of groupoids.
In a topological and operator theoretic setting much has been done, mostly initiated by Jean Renault. For instance, J.-L. Tu used groupoids heavily for proving some forms of the Baum-Connes conjecture.
Not to mention category theory itself and advanced algebraic topology (see e.g. the work by Georges Maltsiniotis and his school elaborating on Grothendieck's approach to stacks), but I'll stop here because I don't like indulging in name-dropping and I've done already too much of it. I didn't mention or link to work on stacks in algebraic geometry (e.g. by Laumon and Moret-Bailly as well as Lurie and Toën, simply because I'm not qualified).
I hope this gave you some impression that there is a lot around and hopefully this answer gives you some leads you can follow.
The single object in the groupoid corresponding to a group $G$ does not really correspond to anything in the group - but you can think of it as a thing which has as its group of symmetries the group $G$.
As an example, the category whose only object is the set $[n]=\{1,\dotsc,n\}$, and morphisms are bijections from this object onto itself. Then the symmetries of $[n]$ are given by the group $S_n$.
For a more subtle example, consider a topological space $X$, and fix a point $x\in X$. Consider the category with one object, namely the point $x$. A morphism from $x\to x$ is a homotopy-class of paths from $x$ to $x$. Composition of morphisms comes from concatenation of paths. The automorphism group of $x$ is the fundamental group $\pi_1(X,x)$, which is a group of symmetries of the based topological space $(X,x)$.
However, if you consider groupoids with many objects all of which are isomorphic to each other, then the different objects correspond to different realizations of the same group. Moreover, each isomorphism between different objects gives rise to an isomorphism between the corresponding realizations of the group.
To make this precise, suppose $\mathcal G$ is a groupoid where all objects are isomorphic. For each object $x$ of $\mathcal G$, let $G_x$ denote the group $\mathrm{Mor}_\mathcal G(x, x)$. A morphism $\phi:x\to y$ defines an isomorphism $G_y\to G_x$ by taking $g\mapsto \phi\circ g\circ \phi^{-1}$.
A nice example of this is the category $\mathrm{FB}_n$ whose objects are finite sets of cardinality $n$ and morphisms are bijections. Then the automorphism group of each object is isomorphic to $S_n$. Isomorphisms between objects determine isomorphisms of their automorphism groups.
Another example is the Fundamental Groupoid of a path connected topological space $X$. Its objects are the points of $X$. The set $\mathrm{Mor}(x,y)$ is the set of homotopy classes of paths from $x$ to $y$. In this groupoid, $G_x$ is the fundamental group $\pi_1(X,x)$ of $X$ based at the point $x$. Different base points result in isomorphic fundamental groups, and isomorphisms are determined by homotopy classes of paths between these points.
Best Answer
Yes, this is correct (for connected groupoids). More generally the classification of arbitrary groupoids up to isomorphism (not equivalence) is the following: every groupoid is uniquely a disjoint union of connected groupoids (up to permutation), and the isomorphism type of a connected groupoid $X$ is determined by the pair $(X_0, \pi_1(X))$ of the isomorphism type of its set of objects and the isomorphism type of the automorphism group of any of its objects.