Steam Trading Cards be the next 10-gallon hats from Valve. If ye understand that line, then the rest a this story will make sense t' ye. If not, then ye have every right t' scratch yer head 'n ask "Why?" t' everything that follows.
In beta starting today, Steam Trading Cards allow crew t' earn Steam rewards for playing games. The current line-up a participating games be Don't Starve, Dota 2, Counter-Strike: Global Offensive, Team Fortress 2, Portal 2 'n Half-Life 2. crew will earn game-specific trading cards that can be collected t' earn "game badges, profile backgrounds, emoticons, coupons, 'n more."
Here's the simplest way we understand it: Create trading Card sets > craft badges > pillage prizes listed above. There's more details on the Steam Trading Card site.