Actually this words is not good to use as a formal greetings as i think...you can only use this those peoples whose in your level i what mean like friends or family.close person..but not to your superior..you cannot say "what's up boss" we you need to use proper words like "Hi" or "Hello boss" this only my opinion.. cowboy....

I think, "what's up" can only mean "hello" if told to a known friend / acquaintance. it could mean otherwise if told to a newly met person and would probably sound rude. :)

"What's up?" can mean "How are you?" But it can also be used to mean the following: "What's wrong?" or "What's bothering you?": "You look sad today. What's up?" or Person A: "My parents said I can't come to your party Friday." Person B: "Why? What's up?" I do not find that males say this any more than females. It's about the same in the US. It is, however, used more by younger people than older people. The word "up" here comes from the phrase "to come up", meaning to "arise" (happen, occur): "Something came up, so I'll have to postpone our date." The question form started as "What has come up?" and got shortened to "What's up?" over time. While you will hear people say "What's up?" to mean "Hello" (and will hear people answer "What's up?" in return), I feel this is a sad state of affairs. It is a breakdown in communication and social fabric. I still make a point to answer this as a QUESTION. The same goes for "How are you?" The answer should not be "How are you?": Person A: "What's up?" Person B: "I'm just enjoying the day. You?" or Person A: "How are you?" Person B: "A little tired, but fine otherwise. And you?" I do hope we can preserve our ability to speak WITH people instead of AT people.

Actually it's a kind of greeting just as say Hello.And the answer is What's up too.Of course it can be used to ask what's happened.

Hello, and what's up can be used together. For example: Hi, Alice, what's up? This expression is more frequently used by male to start the conversation.