>What does it mean to have a project just for the controls? What does it mean a dll from the web controls?
>

I'll take a stab at this:

A .NET application can be made up of multiple projects that form a "Solution". You buy a framework like MereMortals.NET, which consists of individual projects. You create a "Solution" (your application) and add these framework components (projects) into your solution so that you can take advantage of their features.

When you subclass controls, the general assumption is that you will want to reuse them in other "Solutions". The easiest way to do that is to create a separate project for them. That way, when you create a new application all you need to do is add this project to your application as a reference.