It's versatility as a word is what makes it so common. It's generally used as a greeting, but can also be used as an exclamation of agreement or excitement.
The actual origin of this word has not yet been recorded.
Joe: Tony, watsup?
As an exclamation of agreement:
Joe: Isn't that Stacy's Mom?
As an exclamation of excitement:
Joe: Hey, Tony, guess who's got 500gb of porn.
your walking down the street and some fat ugly chick hits on you.... ."oH, AWES!!!"
"dude, that game was fucking AWES!!"