In Quantum Mechanics spin appears as one type of angular momentum. Indeed, in Quantum Mechanics one angular momentum on the state space $\mathcal{E}$ is a triplet of observables $\mathbf{J}=(J_1,J_2,J_3)$ such that
$$[J_i,J_j]=i\hbar \epsilon_{ijk}J_k.$$
From this general definition one deduces that $J^2 = \sum_i J_i^2$ and $J_3$ commute, and then we consider their common eigenvectors $|k,j,m\rangle$ defined by the equations
$$J^2|k,j,m\rangle = j(j+1)\hbar^2 |k,j,m\rangle, $$
$$J_3|k,j,m\rangle = m\hbar|k,j,m\rangle.$$
We then show that $j\geq 0$ and $j$ is one integer or half integer, and for a given $j$, the only possible values for $m$ are $-j,-j+1,\dots,j-1,j$.
Now, from a physical standpoint, spin is one intrinsic property of particles which was experimentally observed in experiments like the Stern-Gerlach experiment, and which should be accounted for theoretically.
The observations then lead to the usual "spin-half", being theoretically defined as the special case of angular momentum $\mathbf{S}$ whose only value for $s$ is $s = 1/2$ and hence we have $m=\pm 1/2$. In that case we usually consider the "spin state space" as the state space $\mathcal{E}_S$ where the set $\{S^2,S_z\}$ is a Complete Set of Commuting Observables and hence has dimension $2$ with basis composed of $|+\rangle = |1/2,1/2\rangle$ and $|-\rangle = |1/2,-1/2\rangle$.
On the other hand we have the so called spin group, denoted $\operatorname{Spin}(n)$ defined as the "double cover of the special orthogonal group $\operatorname{SO}(n)$" such that there is a short exact sequence of Lie groups:
$$1\to \mathbb{Z}_2\to\operatorname{Spin}(n)\to \operatorname{SO}(n)\to1.$$
Now, this definition of the spin group is overly abstract, but I believe there is a connection between it and the spin from Quantum Mechanics. This is suggest firstly by the name of the group and secondly because both spin and the spin group are somehow related to rotations.
As one angular momentum, spin is a generator of rotations, while $\operatorname{Spin}(n)$ is being defined in terms of the group of rotations.
So, is there a relation between the spin from Quantum Mechanics and the spin group? How can we understand this relation intuitively and how this relation is connected to this overly abstract definition of $\operatorname{Spin}(n)$?
Best Answer
In quantum mechanics, the relevant representations of symmetry groups on the space of states are not our usual linear representation, but projective representations on the Hilbert space. The projective representations of a semi-simple Lie group - such as the rotation group $\mathrm{SO}(n)$ - are in bijection to linear representations of its universal cover. For a detailed discussion and a derivation of these facts, see this Q&A of mine. The "intuition" for the appearance fo projective representation is that states really are not vectors but rays in Hilbert space and hence "phases don't matter".
Now, the rotation group in $n > 2$ dimensions has fundamental group $\mathbb{Z}/2\mathbb{Z}$, meaning its universal cover is just a double cover. Therefore, in $n>2$ dimensions, the spin group is by definition its double cover and hence the group we need to linearly represent on the Hilbert space to have a projective representation of the rotation group. Our beloved half-integer "spin" $s$ is now nothing but the number uniquely labeling an irreducible linear representation of $\mathrm{Spin}(3)$ by $L_x^2 + L_y^2 + L_z^2 = s(s+1)$.