A user control is a kind of composite control that works much like an ASP.NET Web page—you can add existing Web server controls and markup to a user control, and define properties and methods for the control. You can then embed them in ASP.NET Web pages, where they act as a unit.

At times, you might need functionality in a control that is not provided by the built-in ASP.NET Web server controls. In those cases, you can create your own controls. You have two options. You can create:

• User controls. User controls are containers into which you can put markup and Web server controls. You can then treat the user control as a unit and define properties and methods for it. Custom controls.
• A custom control is a class that you write that derives from Control or WebControl.
  

User controls are substantially easier to create than custom controls, because you can reuse existing controls. They make it particularly easy to create controls with complex user interface elements.