In slang, some people use "ain't" instead of "am not", "is not" and "are not". Also, in the present perfect and with "have got", "ain't" might be used instead of "has not" and "have not". For example, "I ain't seen him." "I ain't got no money."

As you can see from the last example, "ain't" is sometimes used with a double negative. For example "He ain't a doctor / He ain't no doctor".

However, "ain't" is not recommended usage. In Britain, at least, it is considered "substandard" or "uneducated" English, so when you here it in songs, understand it, but it might not be such a good idea to use it when you're speaking to most people.