I guess technically, "ain't" would be most commonly used as a conjunction of "am not". Examples would be: I ain't kidding. I ain't paying for this. It might also be used when directed at someone else, as in "You ain't going", in which case it would be combining "are not". Realistically though, it's used most frequently only by people in certain regions of the country or of certain socioeconomic statuses. You wouldn't want to use it in any serious, intelligent conversation.

It is true what she said. ain't = am not