Steam is introducing Trading Cards, a beta system that lets you earn rewards while you play Steam games. Similar to Xbox Achievements, the Trading Cards system will add to your "Steam Level," a system-wide XP bar of sorts, but Valve is also giving users the chance to earn rewards through collecting achievements.
Once you've collected all the necessary cards for a certain game, you'll be able to craft a Game Badge, which gets you rewards such as profile backgrounds, emoticons, and the chance to earn coupons for game discounts and DLC. Both Microsoft and Sony have rewards schemes that earn you additional points, but neither are so directly correlated to playing games.
Game Badges can also be attached to your profile name, allowing you to show off your proudest achievements alongside your Steam Level. Unlike Xbox Achievements, you'll be able to recollect the Trading Cards in the same game to upgrade your Game Badge and earn more. At launch, Trading Cards are only available for a limited number of games, including Steam favorites such as Team Fortress 2, Portal 2, Half-Life 2, and Dota 2. Valve hopes to roll out the system to other titles soon, but in the meantime, you can sign up for the Trading Cards beta here, or get more info at Valve's FAQ.