1. very casual way of speaking, like between friends : "뭐해?" which is pronounced "Mwo Hae?" And this phrase can be spoken a lot between friends.
2.There is a lot of formal way to speak "What are you doing ", and one of them is "무엇을 하고 계세요?" which is pronouced "Mu ut eul ha go gye se yo?" But I guess Korean doesn't speak it a lot. As a Korean, asking someone (who are older than me or I don't know him/her very much) to "What are you doing" doesn't feel quite polite.. Please use this words when you are REALLY wondering what he/she is doing.