Answer:   The main difference between them - User Control is a page file with extension . ascx which can only be used within a single application or project But custom controls are assemblies(dll files) that can be used in multiple applications.

Answer:   User controls are easier to create whereas custom controls require extra effort.
User controls are used when the layout is static whereas custom controls are used in dynamic layouts.
A user control cannot be added to the toolbox whereas a custom control can be.
A separate copy of a user control is required in every application that uses it whereas since custom controls are stored in the GAC, only a single copy can be used by all applications.