One of the most contentious issues during the drafting of the General Public License Version 3 was whether to add an obligation to make source code available for software licensed under GPLv3 if it was provided over a network. The GPLv2 only required source code to be provided upon the "distribution" of the program (although the Affero project, with the permission of the Free Software Foundation, developed a version of GPLv2 which included a requirement to make the source code available to any user of a network, rather than just upon distribution). With the rise of service providers such as Google, open source software was being modified and used, but the modifications were not being made available to the community.
Ultimately, the FSF decided not to include such a requirement in GPLv3 but provide this option through an alternative license, the Affero GPL. It also made the GPLv3 "compatible" with the Affero GPL in Section 13 of the GPLv3. This compatability is "hardwired" rather than a natural result of changes to the GPLv3 which permitted compatability with Apache. And the compatability is "one way": GPLv3 licensed code can be linked with or combined with works licensed under the Affero GPL, but works licensed under the Affero GPL cannot be licensed under the GPLv3.
The final version of the Affero GPL is not surprising. It includes the "network use" language which requires that the Corresponding Source of the Affero GPL licensed code (as well as any GPLv3 licensed code which is incorporated into it) be made available to any network user. The critical provision is as follows:
13. Remote Network Interaction; Use with the GNU General Public License.
Notwithstanding any other provision of this License, if you modify the
Program, your modified version must prominently offer all users
interacting with it remotely through a computer network (if your version
supports such interaction) an opportunity to receive the Corresponding
Source of your version by providing access to the Corresponding Source
from a network server at no charge, through some standard or customary
means of facilitating copying of software. This Corresponding Source
shall include the Corresponding Source for any work covered by version 3
of the GNU General Public License that is incorporated pursuant to the
Notwithstanding any other provision of this License, you have permission
to link or combine any covered work with a work licensed under version 3
of the GNU General Public License into a single combined work, and to
convey the resulting work. The terms of this License will continue to
apply to the part which is the covered work, but the work with which it is
combined will remain governed by version 3 of the GNU General Public
The Affero GPL is an important option for companies or projects that are concerned that they will be used to provide services, but the service providers will not provide their modifications to the community. This issue can be important for a company: I worked with Socialtext to develop the Common Public Attribution License which included a network use provision and a very explicit attribution provision. This option may become more popular as more software is provided as a service. I still believe that the GPLv3 will continue to be much more widely used than the Affero GPL. In fact, I have a small bet with Fabrizio Capobianco, CEO of Funambol, that GPLv3 will continue to beat out Affero GPL over the next five years, so keep adopting GPLv3!