It is technically "bad grammar" but if you are watching a lot of English movies and TV shows, you will hear "ain't" all the time. As everybody says, it is probably not a good idea to use it, but in general it is a shortened form of "are not," "am not" and "is not"-- it works for every conjugation of English: I ain't, you ain't, he ain't, we ain't, y'all (you, plural: only used in the South and in certain classes, but you might hear it) ain't, they ain't. I actually do say "ain't" sometimes-- it's a good word to know when you hear it, but it does not sound nice unless you know exactly when to use it.