Texturing..

Greetings. I''m a hardcore DirectX7 programmer learning both OpenGL and DirectX8 in a parallel fashion, and I have several questions for you OpenGL wizards. 1.) How exactly do OpenGL extensions work? How would I do something like multitexturing, and what headers (and libs) would I include (and link with)? 2.) (Direct3D8 question) How did Microsoft intend for me to get data into their new IDirect3DTexture8 interfaces? I''ve seen those cute little D3DX functions, but they don''t do what I want to do (I want to be able to create a texture and edit its contents (and load my own RGBA values)). The SDK docs seem a little sparse, and all of the samples make use of the D3DX utilities (perhaps you may know of a descent tutorial site as complete as this is for OpenGL). Thanks for the help.