Re: Steam Summer Sale
Games "drop" cards. Really, all that means is, launch a game and leave it open for a few hours and you'll find those cards in your inventory. You can play the game if you want, it doesn't matter after launching it.
Badges are crafted from those cards. Badges, as far as I can tell, reward stuff like profile backgrounds and vanity items like that. Alternatively, you can sell your cards on the marketplace for between 11-50 cents.