Overlay problem

Hi everyone! I have a question: I am studying about using overlay surface in DirectX7 SDK docs and meet a difficulty. In mosquito sample, I meet a segment of code that change RGB bitmap to YUV bitmap. Can any one tell me: +)What is YUV? +)Why must I change image from RGB bitmap to YUV bitmap +)How can I change image from RGB bitmap to YUV bitmap 3)I don''t understand very clearly about: source rectangle and destination rectangle in Overlay surface concept. What are they? Overlay surface is an actual surface like Primary surface, Back surface or "virtual surface" because I found in mosquito program that: The mosquito can fly on Windows''s destop alway on top of any other inteface so how can overlay has that attribute? Thank very much and sorry for many question but I really need your help Thanks again