String theory is basically this.. I haven't been following it recently so I don't know what dimension they're at now (they seemed to increase constantly), but let's say 8. So according to this theory, the entire universe is one string. (or more) So all there is, is this string. Everything you see, taste, feel.. TV, beer.. tits&asses.. it's all just our 3d perception of the reality of this string. And this string is 8 dimensional. That is not something you can imagine.

Now, why a string and not, say, a balloon? Because a string has certain mathematical properties we need, called harmonics. Like if you swing a rope around, it'll vibrate. But these vibrations can be an infinite number of big or small. But in the case of a string, its begin- and endpoint are fixed, so there are set rules for the oscillations. Here is an extremely sucky drawing in paint of what I mean:



So, then we have this 8 dimensional string that is oscillating in place.. so how come we can see so many different things? Well, that is because we can see only 3 of its dimensions. (or 4, if you count time.. which we can experience too) To understand this, imagine you are living in 2d, on a piece of paper. You can only see what's on the paper. So I stick my finger through the paper.. then you can see the cross-section of my finger. A bunch of circles and lotsa red, with a white circle (my bones) in the middle probably. And then I shove my whole arm through it.. but what you see is 4 new circles emerging (my other fingers), and then a bigger circle suddenly (my arm), etc.

So as you can hopefully understand from this example.. if you view something in a dimension lower than what it really is - like in this example you view a 3d thing (me) in a 2d plane (the piece of paper) - then it looks all weird and seems to make no sense. So if we take this string that is sitting there quietly oscillating in 8 dimensions, and we dumb it down to the 3 dimensions we can see (+time) then we get.. the world we see now. Tits and asses.. all really just a string.

That's it in a nutshell...

Alternate universes is something different.. it's a theory that everything imaginable exists.. something that we can neither prove nor disprove. So according to occam's razor we have to ignore it.. but it's a nice explanation for the reason why the universal constants are exactly the way they are, and allow the formation of matter (and ultimately, life). Basically all the other possibilities exist too, but there is nothing there where they exist (no matter) so there is noone to observe it. But like I said, this is a theory that can neither be proven nor disproven..