Strings are not described very accurately in popular science, because much of the physics of strings was only understood long after the mathematical theory was somewhat advanced, and an accurate classical analog for the string wasn't available until relatively recently.
The classical analog people often use is a vibrating band of energy, but this is mostly wrong, because strings don't interact by bumping into each other, like rubber bands do. They can only interact in a strange way determined by the consistency of having infinitely many different particles, all conspiring to make a consistent theory. This conspiracy makes it that when strings merge, they do a complicated thing, and the only correct classical analog to this complicated thing is a black hole merger. This wasn't understood very well until at least the late 1990s, so pop-sci has not caught up.
Ordinary astrophysical black holes are big, and complicated, and black, and gooey. They are electrically resistive, their oscillations decay quickly, they are very irreversible in the statistical sense. The string describes the case that the black hole is extended and charged so much that it is in the non-viscous extremal limit, so that strings are not gooey and lossy like ordinary black holes, but shiny and reversible. The one dimensional shiny black holes are the strings, in the limit that these black holes are weakly interacting, so that the lightest ones are light, and consequently weakly charged (because extremality relates charge and mass).
When black holes collide, the oscillations do not combine in a simple way, they combine the way black holes combine, in a strange acausal way that only makes sense teleologically. So you can't say "this pattern made this pattern", at least not in a causal way precisely, you need to describe the whole thing at once. The ripples running along the string doesn't add to the ripple running along another string when they combine, but if the two combine at high energies to make a long string with many ripples running in a statistical way, so that it has a classical interpretation, they combine to an object which is gooey because it has a lot of junk running around on it, and this is no different than a viscous liquid. When the strings cool down again by shooting out other cold strings, the oscillations die down. The laws of combination are not like superposing waves on a pond, but they are described over the entire space-time world-sheet.
Strings are more elementary than black holes, they are small and simple. You shouldn't be intimidated by the above to thinking that the laws of string collisions require you to understand the classical collisions of black holes! This is no more true than saying that describing a collision in the Born approximation in quantum mechanics requires you to solve the much more difficult classical problem of particles scattering around in that potential. The quantum behavior is simpler than the classical behavior in many cases.
The laws of string collisions, for those strings which are lightest at the lowest energies, that is for those distance scales which reproduce our experience, are extremely simple: they just reproduce the laws of Feynman diagrams, so that the particles combine into other particles the same way they do in the standard model. On the string world sheet, these laws of particle combination are the laws of algebraic products of operators, they describe how to expand a product of two operators in an infinite series of a third operator. This process takes a limit where the points of merger of the incoming particles are smooshed close together by a conformal transformation, so that each of their oscillations is no longer distinct, but merged into a combined oscillation a long long time ago (the collision limit is not really a short-distance limit, but a long-time limit). The combined oscillation just looks like an infinite series of particles in the theory, an infinite series of operators on the world sheet which create the oscillation corresponding to sending in one of these particles from infinity.
This is just like a Taylor expansion, except for fluctuating quantities, and the operator product laws are the laws of string merger and vibration-adding. You can't make nothing, because you always have a world sheet, and the addition law is strange, not by the laws of superposition (like water waves, or oscillations on rubber bands) but by the rules of operator product expansion.
String theory includes every self-consistent conceivable quantum gravity situation, including 11 dimensional M-theory vacuum, and various compactifications with SUSY (and zero cosmological constant), and so on. It can't pick out the standard model uniquely, or uniquely predict the parameters of the standard model, anymore than Newtonian mechanics can predict the ratio of the orbit of Jupiter to that of Saturn. This doesn't make string theory a bad theory. Newtonian mechanics is still incredibly predictive for the solar system.
String theory is maximally predictive, it predicts as much as can be predicted, and no more. This should be enough to make severe testable predictions, even for experiments strictly at low energies--- because the theory has no adjustable parameters. Unless we are extremely unfortunate, and a bazillion standard model vacua exist, with the right dark-matter and cosmological constant, we should be able to discriminate between all the possibilities by just going through them conceptually until we find the right one, or rule them all out.
What "no adjustable parameters" means is that if you want to get the standard model out, you need to make a consistent geometrical or string-geometrical ansatz for how the universe looks at small distances, and then you get the standard model for certain geometries. If we could do extremely high energy experiments, like make Planckian black holes, we could explore this geometry directly, and then string theory would predict relations between the geometry and low-energy particle physics.
We can't explore the geometry directly, but we are lucky in that these geometries at short distances are not infinitely rich. They are tightly constrained, so you don't have infinite freedom. You can't stuff too much structure without making the size of the small dimensions wrong, you can't put arbitrary stuff, you are limited by constraints of forcing the low-energy stuff to be connected to high energy stuff.
Most phenomenological string work since the 1990s does not take any of these constraints into account, because they aren't present if you go to large extra dimensions.
You don't have infinitely many different vacua which are qualitatively like our universe, you only have a finite (very large) number, on the order of the number of sentences that fit on a napkin.
You can go through all the vacua, and find the one that fits our universe, or fail to find it. The vacua which are like our universe are not supersymmetric, and will not have any continuously adjustible parameters. You might say "it is hopeless to search through these possibilities", but consider that the number of possible solar systems is greater, and we only have data that is available from Earth.
There is no more way of predicting which compactification will come out of the big-bang than of predicting how a plate will smash (although you possibly can make statistics). But there are some constraints on how a plate smashes--- you can't get more pieces than the plate had originally: if you have a big piece, you have to have fewer small piece elsewhere. This procedure is most tightly constrained by the assumption of low-energy supersymmetry, which requires analytic manifolds of a type studied by mathematicians, the Calabi-Yaus, and so observation of low-energy SUSY would be a tremendous clue for the geometry.
Of course, the real world might not be supersymmetric until the quntum gravity scale, it might have a SUSY breaking which makes a non-SUSY low-energy spectrum. We know such vacua exist, but they generally have a big cosmological constant. But the example of SO(16) SO(16) heterotic strings shows that there are simple examples where you get a non-SUSY low energy vacuum without work.
If your intuition is from field theory, you think that you can just make up whatever you want. This is just not so in string theory. You can't make up anything without geoemtry, and you only have so much geometry to go around. The theory should be able to, from the qualitative structure of the standard model, plus the SUSY, plus say 2-decimal place data on 20 parameters (that's enough to discrimnate between 10^40 possibilities which are qualitatively identical to the SM), it should predict the rest of the decimal places with absolutely no adjustible anything. Further, finding the right vacuum will predict as much as can be predicted about every experiment you can perform.
This is the best we can do. The idea that we can predict the standard model uniquely was only suggested in string propaganda from the 1980s, which nobody in the field really took seriously, which claimed that the string vacuum will be unique and identical to ours. This was the 1980s fib that string theorists pushed, because they could tell people "We will predict the SM parameters". This is mostly true, but not by predicting them from scratch, but from the clues they give us to the microscopic geometry (which is certainly enough when the extra dimensions are small).
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In my opinion you are exaggerating the power of strings. In my perception it is the all pervading harmonic oscillator potential of quantum mechanical potentials taken to a higher dimensional level. You must know that all symmetric potentials have as a first term the harmonic oscillator in the expansion. When in doubt of the form of the potential, approximate it with a harmonic oscillator.
It may be that it is not really simple strings that control the behavior, but again that a string model is a first approximation of the real underlying mathematics just because of it simply appears in any perturbative expansion of a mathematical form.
My second point is that one can never "prove" a physical model/theory. One can only validate it or falsify it.
At the moment there exist no predictions for any string theory model that can validate it or falsify it. Well, partially true, since all models require extra dimensions. So if one could get the so mathematically oriented string theories to give different predictions for the effect on our four dimensions of the extra dimensions, then yes, one could falsify a specific model. This has been done setting limits for the large extra dimensions models already at the level of LHC data, by not finding the thermodynamic jets it predicted, and this has been used to further other models.