Realizing that my previous post might not be enough:
It's not much different from a tabletop game.
A GM posts the scenery and plays the NPC's, the PC's respond and tell the GM what to do.
Dice are usually rolled with
Invisible Castle, accounts there are free and allow the GM to make sure noone is cheating by spamming rolls and picking the best ones.
The biggest differences with tabletop playing is the pace: it'll be slow compared to tabletop, very slow.
The main advantage of this is that, for a lot of people, it makes actual roleplaying easier: players have more time to get in character and think about how their character would act; as opposed to the quick decisions that sometimes need to be taken at a table, which might result in people going with their own gut reaction instead of that of their characters.
It can take a little bit of adjustment to play smoothly when coming from a tabletop, but this comes natural after a little while. Conversations between PC's among themselves or with NPC's tend to go a bit differently. While at a table, it's easy to have a normal conversation between characters. In a PbP going back and forth all the time can make the game go very slow. It's often easier to just spit it out with the NPC, let the players spit out questions and remarks and then respond to them as if it were a normal conversation. In an actual real-time conversation there would be interruptions instead of monologues and assaulting a person with a barrage of questions all at the same time would be rude, but in a PbP it's the quickest way to run a conversation, especially a meet with the Johnson.
