When you play a game, you expect there to be one ending. (No, throwing your controller through the TV in frustration does not count as an ending). Sure, some games do give you a chance to experience different endings based on the choices you make throughout the game. However, even in these games you're limited to a few at best. Not Fallout 3 though, it's got more ways to finish than you can shake a stick at.
According to a recent podcast with Bethesda's Todd Howard, there are more than 200 possible endings. No, that's not a typo, there really are two zeros on that number.
Hopefully you aren't one of those obsessive gamers that must experience every possible ending, otherwise you'll be playing that game over and over. Still, you'll probably want to play through at least a couple of times (provided the game is as good as we expect) so be sure to block off some time this fall when it hits stores.