The Trading Card program was formerly known as 'CookiesTestApp202352' and was renamed on May 10th. The Trading Card system is detailed in a similar fashion to other Steam Community Tools and will be available on PC, Linux and Mac.
Users are already permitted to trade-in certain items in exhange for Steam wallet funding and also have the option to gift purchases to other users.
The Trading Card system may be Valve's attempt to easily identify reliable traders across its various community marketplaces it has been suggested on the Steam forums.
Alternatively, Valve may be looking to introduce trading card games into the Steam architecture. Blizzard is currently developing Hearthstone: Heroes of Warcraft as a spin-off of their World of Warcraft MMO.
Valve has not yet officially announced the Trading Card scheme or when the beta might be available.