Server Control Extensibility
Server controls are a basic building block for creating ASP.NET applications. You can customize existing controls or you can create new server controls.
A custom server control typically extends one of the standard classes found in the System.Web.UI.WebControls namespace. ASP.NET 2.0 server control extensibility could include: more base classes that encapsulate common behaviors, improved designer support, more APIs for interacting with client-side script, metadata-driven support for new features like themes and accessibility verification, and better state management.