are you using VS.Net? If not than what enviornment are you designing in? The code behind file is where your code is (vb, c++, c#), the aspx file is the web page (html) and style sheet is where you can keep your formating options (i've have used these yet) for the aspx file.
you can write all your code in the aspx file using the <% %> delimiters, but it's ulgy and cumbersome.
Jason Meckley
Database Analyst
WITF