Hi Grimmjow,
There's essentially 2 types of PbP here. One is the regular PbP, where DM's post whatever game they're looking to run here and how many/what they're recruiting:
Talking the Talk - EN World: Your Daily RPG Magazine
Typically, it will be either 4e or Pathfinder, and 4-6 players. They usually will list their source materials allowed as well. You'll make PC's and (usually) place them in teh Rogue's Gallery (
Rogues Gallery - EN World: Your Daily RPG Magazine) and the actual game is conducted in the Playing the game forum. (
Playing the Game - EN World: Your Daily RPG Magazine).
The other type is the "living" worlds, which essentially is a shared setting that all the DM's and players involved in contribute too. There are currently 4 Living worlds.
1) Living enworld - 3.5e ruleset - I'm not involved so can't comment too much
2) Living 4th - 4e rules - set in the Transitive Isle homebrew/shared setting
3) Living Eberron - 4e rules - set in the Eberron setting
4) Living Pathfinder - PF rules - in a homebrew/shared setting
These are slightly different as the characters can swap groups depending on available PC's in the "tavern" and DM requirements. If the DM was to disappear (a far too common problem in PbP) then someone else typically steps in to either wrap up the adventure or run it to its conclusion.
As to the nuts and bolts. Typically a DM will set expectations of posting requirements (usually at least 1 every two days or so) in order to keep the game moving. Dice rolling is handled via the Enworld dice roller or an offsite one such as Invisible Castle. Combat actions are noted and then adjudicated by the DM as normal.
One note, is that PbP is like a marathon. It's a slow process, a typical short adventure lasts 3-4 months easily (say what a LFR adventure would be). There's a high rate of attrition that occurs due to the slower pace. It lends itself well to some roleplaying as you can spend a fair amount of time narrating your PC's actions, but again, it's not for everyone.
I believe the Living 4th and Living Pathfinder are the most active ones depending on your preferred rules choice and would be recommendation for starting out.
If you have any other questions, let me know, I personally really enjoy it and PbP is (unfortunately) my only D&D gaming right now.