On May 15th, 2013 the Steam Trading Card system went into Beta. In order to gain access to the beta, users must join the Steam Trading Cards Group on Steam and will then receive an invite based on the time they joined. Being a part of the beta will change the way your profile looks to the new updated style.
As of June 26th, 2013, it finished beta and is now available to all Steam users. Any beta users who crafted a badge during the testing phase receive a "Foil Trading Card" emoticon. Games and features are being added continuously.
When playing participating games users can have a chance to get trading cards to drop. Each game has a fixed number of cards in the set as well as a limited number of drops for each game. This facilitates Valve encouraging people to trade for other spare cards to complete a set. As well as trading for cards users can also sell or purchase cards on the steam community marketplace. Once a user acquires a specific number of cards in a game no more cards will drop. In the case of Free-to-Play games new "drops" (up to the limit) can be earned by buying $9 worth of items off of the valve run store. The exception being if you bought Team Fortress 2 before the transition to Free-To-Play, in which case would give you the limit without having to purchase items.
Booster packs are an additional way to receive cards for a game. Upon receiving the max amount of drops for a game, and when another user crafts a badge, there is a chance for a user to receive a booster pack for for that game. In order to be eligible they must sign into Steam at least once each week.
After collecting a full set of cards for a game, users can combine the cards into a badge that is shown on Steam Profiles. They will receive a game related background for their steam profile, and a game related emoticon for use in Steam Chat / Message Boards / Profiles. A time limited coupon for a random steam game will also be deposited into the players inventory.
All badges, new and old, give users XP, which levels up the Steam Level. When you level up you will receive extra benefits such as profile showcases, extra friend slots and more to be added in the future.
A June 18th update added the chance to get extra cards in "Booster Packs" in order to be eligible for booster packs which contain an extra 3 cards per game users must have exhausted all the initial card drops. After all drops are exhausted the more users log into steam on a regular basis the greater chance they have of being awarded extra cards. Every 10 steam levels a user levels up increases their chances by 20% until they get a 100%+ drop rate effectively doubling the frequency that they receive cards.
Variations / Sets Available
As of June 4th 2013 only one set of regular cards are available for each game. Each game has a rare chance to drop "Foil" versions of cards to users who are Level 10 and above. The cards are represented by a silver foil like border and because of their rarity tend to go for twenty to thirty times the prices on the Community Marketplace that regular cards do. Foil cards however have no actual additional benefits from regular cards outside of their rarity and collectible nature.
On July 24th, 2013 Steam Trading Cards were added to Indie Game: The Movie. This marks the first instance of a movie getting trading cards.
The second non-game to get trading cards is the documentary titled Free to Play which follows three professional gamers as they compete in the first DOTA 2 International Tournament.
There are also mods such as the Portal 2 mod Aperture Tag: The Paint Gun Testing Initiative that have Steam Trading Cards associated with them.