"What's up" is extremely slangy and is usually used as a substitute for "What's going on with you"/"What are you up to?". I wouldn't recommend using it outside of your friends. Also, "what's up" really has so many possible inferences with it, given the circumstances. It could be friendly, it could be spoken in anger when someone feels threatened...just be careful. :-)

when starting a conversation with a stranger; you would use the "formal: How are you". whereas "whats up =informal" usually people tend to use "whats up" with people they are close with or they know.

Hello Vivian, It is not really equivalent and I wouldn't recommend starting a first time chat with "what's up?" . What's up means almost 怎么了 or 有什麼事 in Chinese . "What is going on ? " , " What happened?". You better start a conversation with it when you are talking on a continuous basis with that person or you are proceeding a previous conversation. It is used nowadays as casual manner of starting a conversation; however I think it is not proper to use it for a first time chat. It rather indicates some intimacy with the person you are chatting with. It refers mostly to previous matters discussed ,where you ask for further information about "what happened?" . It could mean also" what are you doing now?" or "what would you like to do now ?" . Its meaning could differ depending on the situation. It is more abrupt and informal in comparison to "How are you ?" "How are you ? " means 你好吗？ and it is the most common way to start a conversation whether a first time or with an old acquaintance. It is a usual greeting asking about the person's overall well being. It is almost like 你怎么样？“how are things with you ?".

It can depend on the situation. - "What's up?" is sometimes meant as a question, but sometimes it just means "hi" (as a question) A: What's up? B: Not much. How about you. (not as a question) A: What's up? B: What's up? A: Wanna go to lunch? -"How are you?" is almost always meant as a question. A: Hey, how are you? B: Good! How about you? A: Alright, alright. Basically, you can use either one as a greeting, although they are slightly different.

A Vietnamese friend said 'wassup!' to me which is a very slangy version of 'what's up?' and it was very funny because it wasn't natural, even though she is a dear friend. Either that or 'what's up' is a peer to peer kind of slang, said to either friends or social groups that feel familiar. There are a bunch of English greetings in the USA like it, such as 'how are you doing?' or 'how are you?' Many are shortened like 'wassup' and 'how ya doon' confuse my friends who are learning English when they encounter them at work. These aren't really questions, so don't be suprised if someone doesn't seem to want an answer. You can say 'fine, thanks, how are you?' but you can also say just about any greeting in response: 'what's up?' 'not much' 'how ya doin'?' 'hi!' 'what's up?' 'hi, mike!' and so forth. Germans do something similar, 'was ist los?' is a common one I learned, literally it means 'what is loose?' Very confusing! A joke response a German taught me is to say 'everything that's not tied down.' To confirm what others said, though - these are not professional expressions or formal ones, although they might be used in some areas like Los Angeles where a friendly connection is being 'faked', assumed when it is not yet true, and it might also be used when there is a connection such as a mutual friend or fellow traveler and so forth.