A server side control is a built in control provided by .Net framework class libraries. This control may be placed on a web form and all its events are handled on the server.

A user control is a .ascx file created by a developer, to combine built in controls to form a new control which will be required frequently for a web site developement. This file contains a '@control' directive at the top. This file can be included in other .aspx files using @Register directive.