Pretty Formalized, and taken seriously (mostly). You can think of it as a Play-by-Post RPG, with dedicated GMs, moderation (ideally), and CP rewards for GMs, and decent XP rewards for characters.
Go to the White Scenario Boards
and read through the publicly viewable scenarios about midway down, as well as the Rules. You can sign up there, or sign up at yuku.com for a more global-preservation of your nick across any Yuku community.
I also strongly suggest that you Subscribe to the Scenario Requests
section of the White Scenario Forums. That way you'll get email alerts whenever a new Scenario request is open, thus enabling you to jump right in if you wish, without having to constantly check the forums.
It's a less-formal version of White Scenarios. Typically one Player will act as GM, but they can also (and often do) play their character. Everybody gets XP rewarded whenever I or Syn get around to scoring the posts based loosely on the WS Scale criteria (in this case it runs from 0 - 20 xp per post, and it's a rough scoring). Scenario Hall Rules
make it reasonably clear.