It means "is not", "are not", "am not" depending on the context. e.g. "I ain't interested" = "I am not interested". "It ain't happening" = "It isn't happening". It's generally to be avoided. It's not rude, it's just that uneducated people use it a lot. It's especially common in the southern states of America. Sometimes you'll hear educated people saying it for effect though (i.e. "low brow"), but unless you're pretty confident it's appropriate, it's probably better to not use it.

Never! It's bad grammar. "Ain't" means "is not" and you should NEVER say "ain't" It will make you sound uneducated.

In response to Steven's answer, I'm not sure if it's a southern American thing so much as it is an urban American thing. I hear it just as much in northern America as I do in the south. It's truly a colloquial expression, so often times people say it out of habit, not out of a lack of education (although the word is generally more common amongst those who have a lower interest in education). Everyone already clarified what it means, but it's important to note that its usage is mostly between friends. I don't think there's necessarily anything wrong with the word, it's just not professional or formal and should be avoided around people whom you are not close to.

I love these questions.

Only properly used in country music!