PMOG stands for Passively Multiplayer Online Game. It works on the idea that you are going to be spending all day online anyway, so you may as well complete some missions and get some points for doing so.
Missions, essentially, are paths through the internet that you can create or follow. Users, or players, think of an idea, and then create a series of links and notes to guide a player through the theme.
I've tried it a bit this morning, and it's fun, if a bit pointless. It does seem like a good way to surf through the interwebtubes on those days when you have nothing else to do.