I love this question because I am from the American South. "Ain't" is a word that means the same as "isn't" and is used regularly by locals in the west and south of America. However, as with all regional English speech, you should be careful about using it. It is best to learn proper English first before using words like "ain't" because there are situations where "ain't" might not be considered polite.

"Ain't" is simply incorrect grammar and is typically used by uneducated people.

The word "ain't " is the contraction of "am not". It is used also as a contraction for are not, is not, has not, and have not. No one uses it in Australia unless they make a clichéd statement such as "Ain't that the truth!" to emphasize agreement with someone's statement.