'Ain't' is not used in standard ('correct') English, but it is a very common word in dialects and 'uneducated' forms of British and American English. It is used as a contracted form of 'am not', 'are not', 'is not', 'have not' and 'has not'.
I ain't going to tell him.
Don't talk to me like that - you ain't my boss.
'It's raining.' - 'No it ain't.'
I ain't got any more cigarettes.
Bob ain't been here for days.
