Apart from the web server controls, user control is a different type of control which can be used to create your own custom, reusable controls using the same techniques you use for creating ASP.NET Web pages.
<%@ Register TagName="ucLeftmenu" TagPrefix="ucLeftmenu" Src="leftmenu.ascx" %>
3. Accessing registered control inside body as webserver control.
<ucLeftmenu:ucLeftmenu ID="ucLeftmenu" runat="server" />