Sort of like 'what's up?', an informal greeting. Can be used instead of saying, 'hi'. They're basically the same. What's happening? What's going on?, What's shaking?, What you up to?, How are you? etc

but there is small difference between what's up? and what is going on? I listened many times people use "what is up" after "hello" so it's sometimes like - how are you?. And what is going on is what is now happening? - you ask when you are curious not about state of health of you respondent or about his mood but about situation which is now in this particular place. But "what is happening" is not widely used, people mostly say - what has happened?