Just the Facts | Programming Microsoft ASP.NET 2.0 Core Reference

In ASP.NET, there are two big families of controls: HTML controls and Web controls. The former group includes controls that are in one-to-one correspondence with HTML elements. The controls in the latter group offer a more abstract programming model and richer functionalities not specifically bound to one HTML element.
If made invisible, ASP.NET controls don't generate any markup code but are activated and processed anyway.
Adaptive rendering is the process that enables controls to generate different markup for individual browsers.
ASP.NET 2.0 controls let you declaratively assign a browser-specific value to properties. For example, you can use one style for Internet Explorer and another one for Mozilla Firefox.
The vast majority of ASP.NET 2.0 controls can generate XHTML-compliant markup. Non-XHTML mode is supported for backward compatibility.
New controls let you fully manage programmatically the <head> tag of a page.
Everything you put on a page is ultimately processed as a control, including literal text, blanks, and carriage returns. Contiguous characters are conveyed to a single control instance.
Validation controls let you test for valid types, values within a given range, regular expressions, and required fields.
Validators let you put declarative boundaries around input controls so that any user's input is filtered and validated both on the client and server.
In ASP.NET 2.0, group validation allows you to specify validation of only certain controls when the page posts back.