There are two ways in which this question is asked: What's going on (with you)? Casual, friendly. The person wants to know what is new in your life. It is part of normal conversation. Responses: "Everything's normal, nothing special." OR tell about something that has happened recently - gone on vacation, saw somebody you both know, just had a test, etc. The second way is factual. What's happening = Explain the situation to me. The person asking has seen a situation that you have just seen (and are possibly still seeing) and wants the situation explained. "What's happening here?" "There was a car accident. One person was injured." "What's happening with Dan?" "He was very sick and won't be back until Tuesday."

What's going on ? is a question to inquire if something is wrong or like " what's happening?" ,but with more emphasis and wondering. So i guess all you have to do is to say what really had happened maybe,with details. that's different from what's up? it's like asking " how are you?" so you can just say "fine". Hope that helped!

i think when someone asks us by saying "what's going on" he wants to know our latest condition, especially the things that happen to us or something around us recently (including our study, family and etc).

thank you so much... @stephanie

For me, if "what's going on" is used in the same context as "what's up" then the very common response might be "not much", or "nothing new". Just like the common response to "how are you" is usually "fine" no matter how the person actually feels. Depending on your relationship to the person, if you have time to talk, or the person really wants to know, then you could give details. But if time is short, you are passing on the hallway or something, "not much" or "nothing new" is sufficient.