Y'all is the contractive of 'you all'
It is plural, refering to two or more people. If you use it to refer to one person you will sound ignorant and any nearby Southern will know that
you are not really from around those parts.
Very common in the South, it is rare to find someone (excluding Florida, and that's only because of all the Yankees
down there) that doesn't use y'all.
Saying y'all makes so much more sense than the ever-popular you guys, you guyses, or even you all. It is much shorter and gets the point across much faster. And takes less breath.
It is never ya'll. The 'a' is not a part of you, it's a part of all (
you wouldn't spell it out as youa ll, would you?).