user controls are reusable component , in that we can use it in many place where ever we want
Custom Control and User Controls in asp.net1. User Control is a page file with extension .ascx which can only be used within a single application. But custom controls are assemblies(dll files) that can be used in multiple applications.2.User Controls cannot be added to the ToolBox of VS.NET . To use a user Control with in an aspx page u have to drag the user Control from the solution Explorer to designer page.But Custom Controls can be added to ToolBox of VS.NET.3 .Since the dll assembly of a custom control is being used,a custom control developed in C# can be used in a project developed in VB.NET or any other managed code and vice versa.This is not possible with user controls.They are language specific.