The only way to address two or more people. Y'all sounds a heck of a lot better than youse guys and is more efficient as well. Only say it if you're from the south unless you want to get laughed at because yankees don't know how to pronounce it right. Ya'll is spelled incorrectly and "all y'all" is perfectly legitimate. Never use y'all to refer to one person--it makes things really confusing.
He was fixing to teach y'all a lesson.
I was wondering what all y'all had in mind.