Ever Heard of Steam CD Keys?
Steam CD Keys are apparently these redeemable codes you can purchase from Steam retailers. It is received as a combination of letters and numbers typically emailed to you as a receipt when you buy a game off of Steam.
When you buy a game for Steam from a different retail store, you will find the CD key labeled on a card, sticker, or disk shipped along with your game. Be sure to check the packaging to make sure your key is valid and can be registered on Steam.