Sometimes it may happen that you need a functionality is not  provided by built in ASP.net controls at that time developers can create ASP.net user controls or ASP.net custom controls.

• 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.

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