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."
Sorry for this long post, but I had to handle my business about my city.
And the "Yo" is the pronounciation of the word "you're," is usually spelled yo'.
"Yo man, that jawn was hot yesturday, yo."
"Yo, I can't believe they ran out of cherry wooder ice!"
"Yo! Come here!"