Now comes the real science question related to string theory: Does it describe our universe? The short answer is that no, it does not. It can be written in such a way to describe some idealized worlds that bear similarities to our world, but it can’t yet describe our world.
Unfortunately, you have to know a lot about string theory to realize that. String theorists are rarely upfront about how far their theory is from describing our reality (when talking to public audiences, at least). It tends to be a disclaimer, woven into the details of their presentations or thrown in just near the end.
In fact, you could read many of the books out there on string theory and, after turning the last page, you wouldn’t have ever been told explicitly that it doesn’t describe our universe.
The world described by string theory has at least 6 more space dimensions than the 3 we know, for a total of 9 space dimensions. In M-theory, there are at least 10 space dimensions, and in the two-time M-theory, there are 11 space dimensions (with 2 time dimensions tacked on).
The problem is that physicists don’t know where these extra dimensions are. In fact, the main reason for believing that they exist is that the equations of string theory demand them. These extra dimensions have been compactified (in some models) in ways that their particular geometry generates certain features of our universe.
There are two major ways of dealing with the extra dimensions:
The extra dimensions are compactified, probably at about the Planck scale (although some models allow for them to be larger).
Our universe is “stuck” on a three space dimensional brane (brane world scenarios).
There is another alternative: The extra dimensions may not exist. (This would be the approach suggested by applying Occam’s razor.) Various physicists have developed approaches to string theory without extra dimensions, so abandoning the idea of extra dimensions doesn’t even require an abandonment of string theory!