Ah, I should add to this. The above answer stimulates discussion and gets people asking you more questions. If that's NOT your intent, then you want to be specifically ambiguous. If you get sick of having the same conversation every single time you mention what you do, or you're playing the modesty thing in a crowd that wouldn't appreciate someone whose "made it", then give an answer that seems very specific but isn't interesting.

"I work in accounting."

"My background is in management information systems."

"I work at a small company coding software."

I don't care for simply "I have a startup", because it stimulates discussion and you get the same questions all the time - what specifically do you do, what field, how long, profitable?, etc. Might as well knock half of those out right away if you want to talk about it.

And if you don't want to talk about it, you're going to need to give someone a specific answer, so when someone's wife asks "What's cperciva do?", your new friend can say, "Oh, he builds computers or something like that."