I think you mean the difference between such examples: "What do you do?" and "What are you doing?", right? Okay, the first example is in Present Simple, the second one in Present Continuous. What do you read? - that means, what do you USUALLY read? What ARE you READING? - Present Continouos expresses "right now" action. Say, I am sitting here right now and asking you about the book which you are reading right now. What do you write? - You can ask a write what his sphere is, if he usually writes detective stories or historical novels or what? What ARE you WRITING right now? - somebody is sitting at the table and writing something, you are curious and ask him or her: What are you doing, what are you writing? is it a letter or just your homework? Don't forget though that a few verbs can't be used in Present Continuous: to be, to like, to think, to love, to hate. It's pretty obvious: it would be strange if you said: "I am loving you" (yeah, yeah, McDonald's doesn't care much about this rule), as if you love somebody in this very moment and doesn't love him a moment later.