If you use classic asp, we create include file in asp. Web user control is a replacement for the include file feature in Asp. Though the model is similar with asp include page, Web User Controls have a lots of new features. You can embed them Asp.Net Web Pages, where they act as unit.
to create Asp.net User Controls is very easy. Open Vusial Studio IDE and then open or create Asp.Net Web Site. Right click on the project ,click add new item and then choose Web User Control select the programming language which you want to use.
Read more about this..