How are you doing? / How are things? / How's everything? / How's it going? ..are all common casual forms of 'How are you?' (and, a special one: Australians mix them into 'How are you going?') :)

How are you doing? ¿Cómo estas? What are you doing? ¿Qué haces?

"How are you doing?" - a way to ask "How are you? ( Keith pointed out other ways too) "What are you doing?" - a question about your actions at the moment.