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.
Can be substituted with:
Bonnie sends her BFF a text: "Guess what?! I just scored a limited edition Simpsons phone case!"
Her BFF replies: "Awes."