1) To refer to someone informally

2) An exclamation to get attention

HEY!

used to draw attention to an object, word or sentance, usually said by yourself.

Commenly assosiated with urban-slang. Though can be used by anyone who lives near or in a state that has a city that people five(min) states away have heard of.
Overdoing it makes you a candyass.
Yo responsably.

another word for hey or hi

is used as a greeting or an exclamation of excitement or frustration.also can mean stop.

Shorts for "your".

YO: a simple word that requires a simple definition:
The word yo does not have one dominant definition there is only one way to describe it's many uses:
1. (YO; as in yo-yo) An exclamation (Can be either negative or positive in it's connotation) meant to capture one's attention.

Used a million times by the next rapper in a circle to let you know it's your cue to stop.