1) pygame.gfxdraw is needed because not all module of pygame are imported by default
2) Surface is where you draw, ... imagine it that you draw in a buffer.
3) To transfer the buffer to the real screen. (to simplify as it s still a buffer)

I suggest you to read the documentation and particulary the examples :
http://www.pygame.org/docs/

And also this beginner tutorial : http://www.learningpython.com/2006/0...game-part-one/