] is act like template as in ASP.NET master pages allow you to create a consistent layout for the pages in your application. where as 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.
In short A Master Page always act as template and the merging container for the web pages and controls whereas a User Control is always a control which can be used again in many pages.