A really nice explanation of AGPL:
Reading AGPLIn a nutshell: GPL is a copyleft license, but GPL doesn’t actually require sharing and licensing source code if you run it for people as a network service, rather than providing it for them to run on their own computers. AGPL was written to close that loophole, often called the “ASP Loophole”, since “ASP” for “Application Service Provider” was the buzzword at the time for what we now call “SaaS”.
Of course, even with it clearly explained, it's hard to interpret, as the author points out.