What is the difference between Web User Control and Web Custom Control?

If your control has a lot of static layout, a user control might make sense. If your control is mostly dynamically generated — for instance rows of a data-bound table, nodes of a tree view, or tabs of a tab control — a custom control would be a better choice.

The main differences between the two types are outlined in this table:

Web user controls Web custom controls
1. Easier to create 1. Harder to create
2. Limited support for consumers who use a visual design tool 2. Full visual design tool support for consumers
3. A separate copy of the control is required in each application 3. Only a single copy of the control is required, in the global assembly cache
4. Cannot be added to the Toolbox in Visual Studio 4. Can be added to the Toolbox in Visual Studio
5. Good for static layout 5. Good for dynamic layout

Read more detailed article here.