A word who's origin is made to be puzzling, but is really made famous by the Movie "Rocky" and by various rappers. Yo is a word deeply connected in the Philadelphia culture, therefore; no, it is not used only by "little black kids" or "wannabe suburbans" like the definitions before it has stated.
A history lesson about the word's popular use in Philly from the New York Times:
"In the 1930's a large proportion of the residents of South Philadelphia were Italian immigrants, mostly from the Campania region of southern Italy, the principal city of which is Naples.
In the Neapolitan dialect "guaglione" (pronounced guahl-YO-nay) signified a young man. The chiefly unlettered immigrants shortened that to guahl-YO, which they pronounced whal-YO. That was inevitably further shortened to yo. The common greeting among young Italian-American males was "Hey, whal-YO!", and then simply, "Yo!" And so it remains today."
1) A contraction of the possessiveprenominal adjective "your."
2) An informal address or title to one whose name is not known to another; can be used as an interrogative address.
3)A declaritive or imperative exclaimation, whether alone or within a sentence.
1) How's yo momma?
2) Hey, yo! What's up, yo?
3) Yo! What the hell do you think you are doing?! Yo, just do your job! YO!