well, what i've done is buy the games from a store or online (Half life 2, cz) and then once there installed, they go on your steam account. Meaning that if you lost the original disks, or want to play it at a friends house, you just login to steam and download your games. But if you buy it thru steam, i would think that you just right click and press "purchase" and then it gets downloaded into your account.