I hear this ain't expression all the time when I watch U.S movies, I have checked the internet but I am a bit confused. Could you give me an example using ain't? And one more thing is it OK to use it during a general conversation?
It has nothing to do with polite or not. It is slang, there is a difference between crude slang and just slang.
Ain't is a southern term and is very commonly used all over the South.
It has to do with how "proper" you are trying to be in your spoken English grammar. In the South it is used in Schools and many other areas of the US. But still they are not speaking proper English when it is used, they are using slang to express that statement.