But there is some confusion surrounding how it works. Here's a conversation with a friend I had this morning that breaks it down for you.
King of Dew: yo
King of Dew: how does the steam card thing work?
King of Dew: i accepted the invite
King of Dew: and used the gift
basementdigital: So you collect all the cards in a game
King of Dew: but there is nothing in my library
basementdigital: You have to play the game first
King of Dew: like i play the game?
King of Dew: ok....
King of Dew: do they drop?
basementdigital: So cards drop based on how many hours you put in
King of Dew: oh
King of Dew: where do they drop?
basementdigital: Like with TF2- all you need to do is idle for a couple hours because you've played it so much already.
basementdigital: They drop to inventory
King of Dew: oh ok
basementdigital: So I got my cards for TF2 last night
King of Dew: then what?
King of Dew: what do you do with said card
basementdigital: But I didn't have each one- so it forces you to trade with friends or on the market
basementdigital: Once you have "them all"
basementdigital: Like a complete set for the game
basementdigital: In TF2's case- it's one card for each class
King of Dew: i've seen that on the market
King of Dew: ok
King of Dew: so.... it's just a preistige thing?
basementdigital: You go to the "badges" section on your profile and craft a badge with your cards
basementdigital: Then once the badge is crafted- they give you all sorts of goodies. Like emoticons http://steamcommunity.com/economy/emoticon/spycon
basementdigital: And discount coupons
basementdigital: Backgrounds for your profile
basementdigital: It's pretty damn fun actually
basementdigital: And all the goodies they give you appear to be tradeable and marketable as well.
King of Dew: i see
basementdigital: It's like POGS for steam. lol
King of Dew: so baically, collect them all... make a badge
King of Dew: get cool stuff
basementdigital: Yup- and with the badge... yup
Have fun out there in gaming land! :)