Valve has added trading cards as a new way to personalize user accounts. Cards have been added added to 30 games already. Up to half of the set are awarded for playing the game. The other half are left for player to trade with.
Collecting cards lets you craft new badges for your Steam Profile and earn rewards like emoticons, profile backgrounds, and coupons.
The cards are grouped into a set for each game. The idea is to collect the whole set which can then be traded in for an achievement badge. Such badges have up to 5 levels which to contribute to the XP level.
During badge crafting you’ll receive random rewards from a set of game related items. They are tradable and can only be used while you possess them.
The XP is calibrated to be harder to reach higher levels with an exponential scaling system. A person with 5000 games might reach level 300 or so with a lot of work collecting cards.
Valve has also created a trading system and market for the cards. Friends can trade cards or they can trade with the game community members. The market is available for those who want to buy or sell cards.
Value has done this largely to help develop a loyalty to Steam so that competitors will not be able to easily replicate the gaming experience that Value has achieved.