What is the difference between the Affero General Public License and the GNU General Public License (GPL)?
Assume the following:
You are developing a server side application in GPL. Now this application serves HTML and not an executable which is directly executed on your machine. That means that another guy could take the GPL code, adapt it and does not necessarily need to publish it. Ie. he can create the identical service using your software without violating the GPL. (Although THEN he cannot publish the software itself i.e. selling)
Not so with the AGPL.
This hole in the GPL is often called "Application Service Provider" hole.
Search for "Why AGPL" or "AGPL vs. GPL" for some real projects who have problems with GPL.
Public web application that uses the AGPL are listed at wikipedia.