LOL. That's what I figured and why I posted this; a lot of people didn't get it, and I could tell looking at inventories.
Some games (and activities during the sale- mostly purchasing games) drop random cards. You get a number of automatic drops per game that varies based on what is available on dlc and what you bought, but you get a maximum of 4 per game. You earn these after you play for a while, and get cards. Then you become eligible for boosters- sets of three cards that are dropped at random. In general, if you collect 8 cards for a game, you get a level in the badge for that game, and steam xps. They have no intrinsic value, like achievements.
But people collect them, and that's where the market comes in, and normal trading. You can trade just like you trade games in your inventory. You can also put them up for sale. If you look in the community section of the steam interface, at the bott there's a link for marketplace.
They go for varying amounts.. From a few cents to a couple of dollars, and it gets credited to your steam wallet. Oh, crafting badges also gets other useless things like emotes and backgrounds for your profile. These can also be sold. That have common, uncommon, rare, and foil cards. The most I've sold one card for is $3, but it varies as the prices go up and down on the market.
