The RPGA is an official network that promotes play of Wizards of the Coast roleplaying games, heavily focused on D&D. With the RPGA, thousands of gamers participate in massive shared-world campaigns in their homes, at conventions around the world, and online. You can often take your character from one play group to a completely different group, and with the RPGA Character Creation Guide you'll know that your character is official. Members also earn RPGA Rewards cards that grant their characters special benefits just for playing games sanctioned through the RPGA. Your home game can also be sanctioned through the RPGA, earning you rewards for your local play.
In order to become a RPGA member and participate, you need a membership number, which can be obtained at a public event (such as a convention or local game day) or online. Go to http://www.wizards.com/rpga for more information on how you can sign up to join the biggest D&D organization in the world.
If you're a Dungeon Master (DM) and you want to sanction a game for RPGA, you'll need to pass a test of basic D&D rules knowledge available on http://www.wizards.com/rpga. The test is open-book and takes 30 minutes to an hour to complete. Once you've passed the test, you'll be able to log in to create events and order adventures.