I'm certainly not an expert but the way it was explained to me one time is that it's like a huge bubble. Not infinite but expanding. What's outside the bubble? I don't know. Will the bubble pop? I don't know.
I believe that the Universe curls back in on its self, like a Mobius Strip. You can travel in one direction, and eventually you'll end up back where you began. However, it's a longer distance each time you traverse the Universe, thus increasing infinity.
Well a simple answer would be this, imagine we some how got to the edge of the universe... What would you see? The answer: more black. So the "universe" is infinite itself but for it to be suitable for things to form.. You would need left over from the Big Bang. Therefore, it is expanding.