It can mean both "wie geht's" and "was geht." It would depend upon when it was used and--I would say--how it's inflected.

You are just meeting up with a friend:

-"Hey, what's up?"

-"Oh, not too much, how 'bout you?"

-"Hey, I'm getting by."

You look a bit upset, so someone approaches and asks:

-"Hey, what's up (Is there something wrong?)"

-"Nothing, I'm just worried how my test went today."

Summarized: One is simply an extended "hi," while the other is asking more like "was geht?" or "what's going on?"

Hmm... to answer your question more specifically: if you were greeted with something like "what's up," you have lots of options for a response. You could either answer the question with a generic response like, "Not too much," or "Nothing," or even ask (basically) the same question in return.

-"Hey, what's up?" (Greeting)

-"Hey, how's it going?" (Response)

"not much" wäre wahrscheinlich die häufigste Antwort (..."nicht viel" oder so). Man würde denken, dass es deshalb "was geht?" betrifft, aber "I'm tired" wäre auch eine mögliche Antwort.

Oft antwortet man mit aktuellen Pläne (z.B. "We're going to the movies", "I'm gonna go drop off a letter at the post office.").

Danke fuer die schnellen Antworten.

Wenn ich mit "Not much." antworte, ist das dann auch nicht negativ gemeint?

It would only be negative if you chose it to be. In other words, it would only be taken as negative if you inflected in such a way.

"Hey, what's up?"
"The sky!!"

;-)

it is close to - how are you - noone really want to know...

translate into

what are you up to? - you might want to give your life's minute plans or otherwise...

;-)

Danke fuer die guten und lustigen Antworten:)

Da kann ich dir nur den Klassiker empfehlen:

http://www.youtube.com/watch?v=MJO3FmmFuh4

man muss sich nur anpassen können...

@ Halen's leider funktioniert das Link nicht mehr.