From the Wizards of the Coast's D&D Encounters page:
D&D Encounters is an exciting, weekly campaign that plays out one epic encounter at a time. As you defeat enemies, solve puzzles, finish quests, and perform heroic deeds, you’ll earn Renown Points that you can use to get exclusive rewards. Each session only takes 1-2 hours to play, so it’s easy to fit your game in after school or work. And each week there’s a new and exciting challenge. Jump in anytime!