User controls are an answer to prayers we have been secretly been petitioning the web gods about. Include files have always been a semi-answer to simple code reuse in web pages, but there are several inherent problems with include files that have been powerfully addressed by User controls.
User controls are essentially reusable blocks of code focused for user interface purposes. They are available to reuse as common components across your web application. They aren’t magic or really any different than ASP.NET pages, except that they are created and formatted in a particular way so that they may be reused across your application.