Question - What do you mean by user control and custom control in C#?
Answer -
User controls are very easy to create and are very much the same as the ASP control files. You cannot place a user control on the toolbox and cannot even drag-drop it. They have unique design and individual code behind these controls. Ascx is the file extension for user control.
You can create custom code as the compiled code and can be added to the toolbox. You can include these controls to the web forms easily. Custom controls can be added to multiple applications efficiently. If you want to add a private custom control then you can copy it to dll and then to the bin directory of your web application and use its reference there.