What are the basic differences between user controls and custom controls?

Now that you have a basic idea of what user controls and custom controls are and how to create them, let's take a quick look at the differences between the two.

Deployment::::::::::: Designed for single-application scenarios
Deployed in the source form (.ascx) along with the source code of the application

If the same control needs to be used in more than one application, it introduces redundancy and
maintenance problems Designed so that it can be used by more than one application

Deployed either in the application's Bin directory or in the global assembly cache

Distributed easily and without problems associated with redundancy and maintenance

Creation::::::::::::::: Creation is similar to the way Web Forms pages are created; well-suited for rapid application development (RAD)

Writing involves lots of code because there is no designer support

Content:::::::::::::::: A much better choice when you need static content within a fixed layout, for example, when you make headers and footers

More suited for when an application requires dynamic content to be displayed; can be reused across an application, for example, for a data bound table control with dynamic rows

Design::::::::::::::::: Writing doesn't require much application designing because they are authored at design time and mostly contain static data

Writing from scratch requires a good understanding of the control's life cycle and the order in
which events execute, which is normally taken care of in user controls

REFERENCE::::: http://support.microsoft.com/kb/893667