To add a custom control to a Web form what is needed?

281.

TagPrefix

Name space of the dll that is referenced

Assemblyname

All of the above

You can make your custom server controls available in the Visual Studio Toolbox by defining the appropriate design-time attributes to be used by the visual designer

You can do:
<%@ Register Assembly="ServerControl" TagPrefix="aspSample" Namespace="ServerControl"%>

or in the web.config:

namespace="ServerControl">