I guess now I'll look up what a Steam badge is...
Well you collect a full set of cards from the game (usually there's like 8 of them) and you get a badge and a special smiley you can use or something
While you are playing the game, cards will start get added to your inventory at random, up to half of set (so for example, four cards out of eight).
The other four you will need to trade for or buy to complete the set
Or if you are not interested in the whole cards thing, you can just sell them on the steam market
