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