The five new games are:
Guns of Icarus Online Jagged Alliance - Back in Action Zombie Driver HD Offspring Fling Magicka
The Steam Trading Card game is a virtual card collection game where cards are earned by playing games on Steam. These cards are eventually combined into game badges.
The game is still in beta, with new games added regularly. Half of the cards you can get are “dropped” throughout gameplay–which means the more you play, the more likely you are to get drops. The other half is earned through collection and trade. Once you get a set of cards, you can craft them into game badges. Badges are then tied to and displayed on your Steam profile, and can be traded in for emoticons, profile backgrounds and coupons.
Other games on the Steam Trading Card game are Borderlands 2, Bit.Trip Presents Runner2, Brutal Legend, Left for Dead 2, Monaco, and Team Fortress 2 as well as many others.
To become a part of the Beta, you simply have to join the Steam Trading Cards Group on Steam. Once joined, you will be sent email invites as well as some extra invites to give to friends.