It can mean am not. I ain't going to the store for you.
It can mean don't you. Ain't you got no sense?
It can mean is not. It just ain't happening.
All of this, of course, is very bad United States English spoken by the most ignorant of people.
However, it is my understanding, that in England it can be used as a very posh way of speaking used by the upper class. I don't quite have a handle on that one but I am sure an English person can expound on that one and give examples.