Master Pages are used to provide the consistent layout and
common behavior for multiple pages in your
applications.then u can add the Contenetplaceholder to add
child pages custom content.
User Controls:
Sometimes u need the functionality in your 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 controls are
those with .aspx extensions and u can share it in the
application.