A lot of web developers while learning ASP.NET get confused with the difference between an ASP.NET Page and ASP.NET User Control.
A User Control is a group of one or more ASP.NET Server Controls or HTML Controls that can be reused. So for example if you need to create a Login Control, you would group a bunch of TextBoxes, Label and Button Controls and create a User Control out of it. This User Control can then be reused on multiple pages, providing a Modular approach to web development.
User Control are similar to ASP.NET Web Pages (WebForm). User Control can have markup, code and script, just like Web Pages have. Since both the User Control and ASP.NET Web Page inherit from the TemplateControl class, they also share common methods and events.
However there are some basic differences between the two: