ASPX is the design code ASPX.Cs is the code behind server code
The codebehind in 1.x was a "complete" class. It would extend the "page" super class, and have all the controls for declared and added within that class.The 2.0 model uses a "partial" class which allows you to split a class over a few files. this way you dont get the "designer generated code" clogging up your work area. Quite a nice idea actually, it keeps things nice and tidy. Here is a more indepth explanation...http://davidhayden.com/blog/dave/archive/2005/01/19/783.aspx
code behind class main function.. aspx is just apperance
Analogy: You can think of ASPX as the human face and skin and the code-behind (ASPX.CS) is the brain and other internal organs that signals your body what to do.
ASPX inherits Code behind class.