User control is like other control which can be designed on basis of our own requirement to avoid the repeating of same code in entire application. It will increase the productivity and maintainability.
For example, if we have to design same registration page in two module of application then we can create the user control for one registration page and we can reuse this user control in other page.
How to implement user control in asp.net web form page ?
Firstly you create the user controls like normal web form page, only page extension will be different i.e .ascx
You can add the user control in your page by using Register page directive like this image.
TagePrefix: Indicates the namespace that you want to associate with the usercontrol for the current page
TagName: Indicates the name that you want to associate with UserControl for current page.
Src: Indicates the virtual path of user control.