Roleplaying games are filled with dice. At some point, you might want to know what your odds really are. Perhaps you're a player, perhaps you're writing a game yourself. In any case, you'll find there's a lot of math involved behind the scenes.
I created AnyDice because I grew tired calculating probabilities in my head all the time. Sure, the simple cases aren't a problem, but stuff like exploding dice just make my own head explode. I'm sure the same goes for most other people, and I like to save you all from a headache.
It's fun to actually compute things yourself the first time, but soon it becomes a chore. You know how to do it, but you also know it's going to take a while. You no longer want the mental exercise, you just want answers. Enter computers! Those things are simply far better at counting than any human could ever hope to be. Add the Internet, and you can get your answers anywhere, as long as you're online.
AnyDice isn't very smart, it just counts and counts until it's finished. That's fine for most cases, because it's really fast. Sure, it uses some tricks to speed things up, but some problems just scale too fast. Luckily, most people won't have to worry about hitting the ceiling anytime soon. For those that do bump their heads: I'm working on it!
AnyDice is a server-side process, not client-side. Why? I've seen Javascript-based calculators. Input something meaty, watch your browser choke and die. That's why. The worst that'll happen with AnyDice is that you'll have to wait a while, until the server kills the process because it took longer than half a minute. Then you'll get back half a page, which is useless and ugly, but at least it won't lock up your browser. It won't hog your CPU, but the webserver's. That might become a problem if this site gets lots of traffic, but that's not the case right now.
I'm not finished with AnyDice yet. There's still lots of room for improvement, performance to gain, and features to add. I'm also working on spin-offs that reach into the iPhone and desktop fields, not just online stuff. Check out the Blog to see what I'm up to.