Wow. That is rather like asking what pi is used for. e is a mathematical constant that "pops up" repeatedly in advanced maths.
Perhaps the most obvious use is in calculating compound interest when that interest accrues continuously.
It's also used in probability theory.
If you ever get deeply into calculus (it's great fun, honest) you'll find that e rapidly becomes your friend.
There is a lot of mathematical "jargon" out there on the subject (and the wiki entry, whilst correct, is particularly unhelpful as an explanation, unless you are already at a pretty advanced state).
