You should always use ' isn't '. ' Ain't ' is poor English spoken by uneducated people; the only time to use it would be if you were acting such a part in a drama, etc.

Of course it helps, Alyssa. Thanks a lot.

They mean the same thing. But ain't is slang. Its not a real word. Always use isn't.

It means am not/are not/is not or has not/have not. I'd also like to add that "ain't" isn't considered standard English, though I wouldn't say it is only used by uneducated people haha! It would be used in an informal setting, and I have found that it usually depends on someone's accent/dialect. Hope it helps! :)

Thanks, Giulia. Examples are always helpfull. :)