Play-by-post, essentially. There's a forum for the extranet site that all the characters frequent (where everyone just posts as and when they want, basically it's just like here
), and another forum for the actual plotlines. Players start their own arcs, or get together in groups to do so. Some threads are open, and anyone whose character could realistically show up can, others are arranged between whoever's contributing to a given arc. Plus of course arcs overlap, the extranet forum and the plots reinforce each other, etc. Everyone follows as few or as many characters/arcs as they're interested in.
There's a shared fanon that everyone agrees to, naturally, which is now playing in a post-Mass Effect 3 setting, so of course they had to clarify what happened to the major factions, etc. There's a series of articles on the site that explain the current state of things. Other than having to conform to the shared vision and the various forum rules, there's a lot of freedom. I'm greatly enjoying myself (as you can probably tell...), and since they're eager for new members I thought I'd see if I can lure anyone in. The site's been up and running for several years, but after playing through the events of ME3, there was a reboot/timeskip, perfect for newbies like myself to join.