=D so glad you asked! i'll be happy to help. ^^
online gaming here at penandpapergames generally takes one of two formats. there is rp via chat, and rp via play-by-post. since (almost) all of our games are tabletop-based, we use various softwares to emulate that.
rp via chat: everyone uses a chat client, whether it is the flashchat here at pnpg, yim, msnm, irc, icq, aim, or something else. at the risk of stating the obvious, everyone uses the same chat client. everyone takes turns typing in what they would otherwise be saying around the table. pros-real time, fairly quick paced. cons-still have to sync schedules, not as fast as around the table.
getting fancy: some people use voice chat instead. this speeds things up a bit, especially where some members of your group(s) do not type very fast. not as good as face to face, but it works in a pinch. and if everyone has enough bandwidth, then you can step that up one more notch and set up a video chat. pros-almost as good as round the table, quick. cons-can't share snacks, still have to sync schedules, almost but not quite.
more getting fancy: shared whiteboard programs, like gametable, allow people to have real-time shared maps that everyone can change and everyone can see when someone does something. great for games where determining where everyone is in relation to what is important. being able to see a map enhances play. there are many different programs to choose from. some are free like gametable, others are not, but have some very nice features.
rp via play-by-post: the dm contacts farcaster here at pnpg. farcaster will set up a special sub-forum where the dm is the mod over in the play-by-post section. everyone posts what they would say. pros-takes very little time out of a given day, and there is no schedule pressure. great for those groups who can never all get together at the same time. cons-S.L.O.W., one round can take a week or more depending on how many players there are.
maps and the like are typically done via in-line or externally hosted images.
i hope this little overview clears things up a bit. feel free to pester me with more questions if you need to. =D