There are several types in action.
The type where everyone makes up a char and a stat sheet according to the creators blueprint or example (creator can go by GM for Games Master) then they all send their 'Plans' the the GM who reads them all and decides the outcome based on planning and creativity. Like Puppet Master.
The type where actions are posted in the thread and players and the GM simply respond to each others posts to move the story along with the GM occasionally putting in set pieces. Like The one Aman is running at the moment (Hall of Heros or something... sorry Aman but I forgot its name).
Some are just interactive choose your own adventures where the creator posts a page of info or story then gives options for people to vote on which one to choose. There is one of these running too, Blackwater... something, if I get to edit this I'll look up the names.
There was even a tower of riddles where the creator simply posted riddles to players (with a little fun back story as well) and players simply had to answer them correctly to advance in a race to the finish.
Basically if a member thinks up a great idea for a game they can either play with, or run for others, they create it and ask for players.