Valve introduces Steam Trading Cards for community rewards
Steam users can earn the virtual cards by playing their favorite games; the way you acquire them varies and is partially community-based. Collecting trading cards allows you to craft sets into badges for your Steam profile or redeem them for tradeable Steam community items. Badges earn players rewards like emoticons, special profile backgrounds, and game or downloadable-content coupons.
You can gain some cards naturally through drops in-game, but these are different from achievements, an FAQ clarifies:
Trading Card drops are currently just tied to playtime within a game, independent of which achievements you are getting. We will definitely look into adding more rules for how cards are dropped in the future, but there are a few problems with in-game achievements that prevented us from using them right now. Many games have achievements that are grindy or un-fun to get, and we want the act of getting game cards to be fun for all users in games. We also don't want to push users to cheat or use achievement-unlockers in games in order to get cards, since that devalues achievements and isn't that fun either. Achievements are still valuable, and having the Achievement Showcase on your community profile is a great way to show off those rare achievements.
So far, more than 30 games make use of the cards, including Team Fortress 2, FTL: Faster Than Light, Grid 2, Monaco: What's Yours Is Mine, Trine 2, Super Meat Boy, and Counter-Strike: Global Offensive. More will roll out every week.
Valve has also tweaked everyone's profile to better reflect what they play and how they participate with the community. Profile "Showcases" allow you to display favorite games, items up for trade, achievements, and more.
In addition, your "Steam Level" is now visible. Every badge you earn contributes XP to this member rank, and higher levels grant more friend and Showcase slots, among other perks. Players unlock new badges by participating in events and sales, collecting trading cards, testing Steam Community features, and so on.