It's a collectible card feature added on Steam. Developers make several background images for use on people's profiles, several small pixel-art emoticons for use in Steam chat and discussion posts, several images to be used on the actual cards and several badge images. Cards drop into players' inventories as they play the games. The cards can then be used to craft the said badges that give Steam XP, one emoticon and one background.
I am sure Grimrock fans playing through Steam would love to showcase the game on their profile and in chat/discussions using the emotes, art and badge.
A bunch of indie developers have already hopped onto the bandwagon but at least 50% of the games taking part are rather... underwhelming in quality and don't exactly have a 'fanbase'.
All developers who have a product on Steam are invited to take part and can do it, provided they send steam the art required.
You can read more about the whole thing here:
http://steamcommunity.com/tradingcards/