'Outta is a colloquial way of saying 'out of'. 'It's time we were outta/ out of here.' 'Ain't' was originally a contraction of 'am not' but it is also used for 'are not' 'is not' 'have not' and 'has not'. 'You ain't seen nothing yet.' is a commonly used phrase with 'ain't' but it is grammatically wrong as it contains a double negative! The use of 'ain't' actually dates back to 1778. Although many people use 'aint', it is considered bad English by teachers and many well educated people.

Jaqui's covered it. :) I'll just add a couple of caveats. Using either of them makes you sound as if you had a poor education, or are just too plain lazy to write properly. This also goes for "wanna", "gotta", and so on. For Urbandictionary, I find too many faulty definitions there. Again, be careful. The problem is that anyone can write an entry.

http://www.urbandictionary.com/define.php?term=ain't http://www.urbandictionary.com/define.php?term=outta That's a great site.