Earlier quoted context omitted.
(Ironically, web apps are an area where you can generally get away with not knowing anything about open source licenses, since you're not redistributing the programs you make.) Except if you happen to use something under an Affero GPL license, in which case you really should know about it. But yes, those project are rare.
I hadn't heard of that one. Is it actually enforceable? I was under the impression that the GPL didn't require opening server code, not because it was missing something, but because server code doesn't get distributed and thus doesn't fall under copyright limitations in the first place.
It's fairly uncommon, but those projects licensed with this version have specific function built in that allows any user to get a copy of the source code (and you couldn't remove it without breaching the license); and some known names use it (MediaGoblin, MongoDB, Diaspora, Ghostscript, Launchpad, POV-Ray are some examples).