The Priests' Guild is centred, if it can be said to have a centre, in the temple quarter in Ankh-Morpork. Each god has their own temple, however, with the more popular gods having temples and shrines in many towns and villages. The god you choose to follow determines your alignment requirements, your rituals, and how much people will be bothering you. Pishites, for example, will be harassed for many, many things, from ressurecting dead players, to status restoration, to even curing drunkedness. Sekkites – and for that matter pretty much every other religion – won't be bothered too much.
As a Priest, your abilities will come in the form of rituals, which are rather like spells except that they use the faith skill tree. The god you choose to follow will influence which rituals you may learn. The faith skill tree also has its own set of commands you can learn as well.
Priests are awarded different titles they can use depending on their guildlevel. A (currently incomplete) list is below.
2008-09-08
2007-07-10
2007-07-09
2007-05-19
2007-05-17
2007-05-16