Gatecrash will be the second set in the Return to Ravnica block for Magic: The Gathering.  The new set will be released on February 1, 2013.
The new expansion set will include 249 cards, featuring the Orzhov, Dimir, Gruul, Boros, and Simic Guilds.  Combined with the Return to Ravnica set, all ten Ravnica guilds are featured.  The action of this block centers around the on-going conflicts between the guilds.  The story will climax in the as-yet-unnamed third set in the block.  During pre-release events for the expansion, players will choose which guild to support, receiving special guild-related promo items.
The cards will be available in Booster Packs, Intro Packs, Event Decks, Booster Battle Packs, and Fat Packs
To support the new release, Wizards of the Coast will sponsor pre-release events on January 26 and 27.  Pro Tour events will be held on February 15 to 17.  The final qualifier rounds for this event are being held on December 16.  A Gatecrash Game Day weekend is scheduled for February 23 and 24.