In this sample chapter from "Pure ASP.Net", we'll see what web forms are, how the code is processed on the server and how client-side code can also be integrated into web forms...
At the most basic level, an ASP.NET Web form looks remarkably like a regular HTML page. It is a text file that contains markup language that can be edited in a simple text editor such as notepad. However, an ASP.NET Web form has several "distinguishing elements, which are discussed in this chapter:
Web forms contain blocks of code that are processed on the server.
The first time a Web form is requested, the entire page is compiled. Subsequent requests are served from this compiled page.
Web forms can contain page directives, which are global settings for the page. Common page directives enable you to turn off session state and ViewState management.
Web forms can contain both server-sides, including SSIs, which enable you to dynamically insert the contents of a file into your Web form, and user controls. User controls are object oriented and use a more programmatic approach to code encapsulation than SSIs.
As mentioned previously, Web forms can also contain as much HTML and client-side script, such as JavaScript, as desired.
ASP.NET 
Reduce Repetitive Code with ASP.NET Custom Controls
