Steam Trading Cards are virtual cards in Steam that can be earned by playing the associated games. Collecting a full set will let you craft a game badge, giving you XP, and the chance to earn profile backgrounds, emoticons, and coupons.

Steam Trading Cards are virtual cards in Steam that can be earned by playing the associated games. Each person gets half the number of cards in a set via free drops, with a chance for booster packs every week that they're active on Steam. Spending money in in-game stores, buying them off of others, or getting lucky with booster packs will let you collect a full set, letting you craft a game badge, which gives you XP, and the chance to earn profile backgrounds, emoticons, and coupons.

They were first introduced in 2013.

http://steamcommunity.com/tradingcards/