Steam Trading Cards, Valve's virtual collectibles that can be traded with other Steam users and spent for various in-game goods. Collecting cards allows Steam users to craft badges for their Steam profiles and "earn rewards like emoticons, profile backgrounds, and coupons."
Steam Trading Cards entered beta back in May, supporting a handful of Valve games and Don't Starve. Right now, more than 30 games support Steam Trading Cards, including Team Fortress 2, FTL, GRID 2, Monaco, Trine 2, Super Meat Boy and Counter-Strike: Global Offensive. Valve promises more games will support Steam Trading Cards with each passing week.