Online, you enter into a lobby where people host games. There you can join games to play… or watch if it has already started. These games are live.
Play By Email… well, of course you’ll need the emails of others. I think everyone has to register to the dice server, TripleA should tell how to do it. But you get spammed a lot from the results of the game. So I’d recommend using a new email just for this if you decide to play it by Email.
Games by Forum or Email aren’t live of course. You get the results uploaded and then when it’s your turn, when you’re done, you post/send your results. TripleA conveniently does most of the work itself for that, I think.
And if you got a buddy right by ya, you could just play a local game.