Web User control or the .ascx file in asp .net is a replacement for the include file feature in ASP. It actually is a positive evolution from the asp model. Classic asp had used .inc file and included it wherever we needed it. Though this model is also similar, Web User Controls have a lot of other features added along with them. Web user controls are derived from System.Web.UI.UserControl namespace.

These controls once created, can be added to the aspx page either at design time or programmatically during run time. But they lack the design time support of setting the properties created along with the control.

They cannot run on their own and they need to stand on another platform like an aspx page. Even if we try to load it on a web browser, IIS will not serve the files of the type .ascx.