Living Greyhawk is the RPGA's single most popular campaign, and boy is it ever popular. The RPGA does not release their play numbers, but judging from the number of events I see at conventions and number of messages on the boards from various campaigns, I can truthfully state that this campaign sees more play than all other RPGA campaigns combined.
The Living Greyhawk campaign is set in the World of Greyhawk, which was one of the original Dungeons and Dragons campaign worlds. Different areas of the world are assigned to different areas of the Greyhawk world and they all get their own adventures that can only be played in their area. These adventures are called regional adventures. There are also other adventures that can be played anywhere and those adventures are called core adventures.
Each region is administered by a Triad, consisting of three gamers who oversee the development, plotline and players in their region. This volunteer system seems to work far better than the campaigns that are directly controlled by Wizards of the Coast.
The Living Greyhawk campaign is currently on its seventh year of play and the world has developed quite a bit in that time. In fact it is developed so much that some players say that it really isn't all that recognizable as Greyhawk anymore, other than the place names. The reason is that simply too much history has happened in the seven years of the campaign, meaning that almost every area in the world has seen major changes from the original World of Greyhawk products that were published from 1980 up until the early 1990s.
The main reason a person might want to play in the Living Greyhawk campaign is that you can play in it almost anywhere, you can quit playing it whenever you want, and pick back up whenever you want, and not be left behind, as the campaign has always supported a wide range of character levels. As of this writing the main campaign supports characters up to 15th level and some special events support characters from 16th to 19th level.
The ability to enter and leave the game at will makes it especially popular with older players who often have families and responsibilities that don't allow them to simply play every Friday night with their buddies like they might have back in college.