Ain't is typically not accepted in formal writing. Ain't is just like aren't, isn't, etc. Some people will say, "I ain't going to school" instead of "I am not going to school." It is pronounced like the word paint without the "p". (AY-EENT)

Ain't is considered a word used by hicks, slaves, uneducated etc. Best is not to use it unless you are trying to convey a special meaning or circumstance. I use it sometimes to express a sentiment of sarcasm etc. For example, in the US, "ain't no way Jose" is considered derogatory meaning, "no way am I going to do this." "Ain't she tweet" as a slur to say to a girl. It is considered as fresh but it is also a title to a famous US song. My advice is that until you have learnt all the nuances of proper English before you try to wade into these shark infested waters.

When I was a kid, people used to sing a rhyme that goes, "Ain't ain't a word, and you ain't supposed to say it. Say ain't five times, and you ain't going to heaven." Really stupid, I know, but I couldn't resist when I saw your question. To be honest it's just like what Lauren G. said, it replaces aren't, isn't, am not, etc.

Best not to use it unless you want to alert people to the fact that you can't speak proper English.

It's Cockney dialect (London). Unless you're from London I would stick to standard English:)