difference between User Controls and Master Pages

Master pages are used to provide the consistent layout and common behaviour for multiple pages in your applications. Then u can add the Contenetplaceholder to add child pages custom contenet.

User Controls:Sometimes u need the functionality in ur web pages which is not possible using the Built-In Web server controls then user can create his own controls called user controls using asp.net builtin controls.User controlsn are those with .aspx extensions and u can share it in the application.