Server controls are the heart of everything that makes developing a Web form easy. These classes give us a programmatic way to manipulate HTML and its interaction with users. Server controls more than anything differentiate ASP.NET from many other development platforms for the Web.
At its most basic level, a server control renders HTML to the page sent to a user. In more complex situations, a server control may render a ton of HTML and provide form elements that it can interpret on postback.
Although many server controls are built-in to ASP.NET for your use, you can also build your own. The implementation can be entirely your own, or it can be derived from existing controls.