Sup is something cool people say. It can be used as a greeting. People say "sup" when they don't feel like saying whats up because it's to long. Also, people say "sup" because they get in the habit of saying "sup" so they just always say it. It can be used by girl or boy.

A term that cool people use because they are too damn lazy to say What's up. (Too much effort involved there man.)

A contraction of the phrase "what's up?"

Short for 'What's Up?'. 'What's Up?' is short for 'What is Up?', and 'What is Up?' is short for 'How are boring daily events in your life going?'.

A greeting, an abbreviation of "What's Up"

How everyone starts an IM convo after saying hey. It is then always followed by "nm"

The "point" to a "sup" is one trying to say hello, sup was derived from wassup, which is another variation of What's up. What's up isnt an inquiry but an informal way of saying hello, although it can at times be answered with an actual response (Ex 1.)
Many do in fact actually respond to the question with an answer but quickly disregard it and fray away from it... another way of responding to it would be (Ex 2.)