In more recent times the word has spread into hip-hop or gangsta culture. A parody of such usage forms the basis for a comic exchange between the animated characters played by Robert DeNiro and Martin Scorsese (Shark Tale, 2004), which can be viewed here (http://yoism.org/index.php?id=YoST/).
Members of this later cultural scene may claim its origin as theirs. The word's origin, however, clearly predates these recent uses, as is evidenced by the words of Rocky Balboa (played by Sylvester Stallone)
"Yo, Adrienne, it's me, Rocky..." Rocky Balboa (Rocky, 1976)
"Yo, Adrienne. We did it!" Rocky Balboa (Rocky II, 1979)
If someone accidentially bumped into you, the expression "Yo!" could be interpreted as "Hey! Watch it," or "What the hell!"
If someone did something that amazed you or shocked you, the word yo is like laughing or just an expression of amazement.
Otherwise, see Lili2344 for a history lesson on where the slang originated.
Hey yo, Joey, wutsamatta. I ask'ja ta trow me the ball tree times already.
African American colloquialism: (now used by all ethnicities despite class, education, and geography)
Yo! Rick baby, what's with yo ride yo?