The CD Key is a serial number with a combination of 13, 15, 18, or 25 letters and numbers - it can be found on a sticker inside your game's case or printed on the game's quick reference card. For authorized online resellers where no physical media is shipped, the CD Key is usually included in an emailed purchase receipt. For questions on who is an authorized reseller, please contact the publisher for a list of their authorized resellers.
The CD Key acts as your proof of purchase for the game - Steam Support may ask for it if you need to establish your ownership of an account. It is recommended that you keep your CD Key in a safe place to ensure the security of your account.
All retail Valve games and a number of third-party games can be registered to your Steam account. All of the following games can be registered through Steam:
Opposing Force CD-keys cannot be registered through Steam. Opposing Force was included in a retail Half-Life package. The Half-Life CD-key included in the package will unlock all of the games in that package, including Opposing Force.
Games which cannot be registered through Steam may still be added to your game list for easy access - please see the Adding non-Steam games to My games list topic for more information on this process.