The fop/asterisk user is for FOP - you can use it to play with but it is used realtime by the Flash Operater Panel (FOP).
You need to read up on the Asterisk manager interface, which is basically a serial TCP interface over which you can send and receive Asterisk commands under program control. In its simplest form you can define a user, telnet into it, and manually poke commands through. It runs at port 5043 or some such (can't remember exactly - look it up on voipinfo).
The entries in manager.conf are a bit like extension entries in extensions.conf. They each define a user who can log-in. FOP is one which we created for the FOP (Flash Operator Panel) application, FOP gets all of its information by poking asterisk via the manager interface.
You don't actually need to know how it works in order to set up a user (although it helps with the setup if you do), you just need to know the setup syntax, which you can find on voipinfo.
Best
S