Buy Prince of Persia: The Two Thrones

Uplay is Ubisoft's Download Platform. If you still don't have the client, you can download it HERE, install it, and create a Uplay account if you don't have one already. Then you will have to get the Download Manager related to your game, unzip its content and use the .exe file which will download your game. At last, once the download is over, you just have to get back on Uplay where you will see your games cover and click it to activate your CDKey.

GOG GALAXY is a fully optional client to install, play, and update your games. It also offers online multiplayer, achievements, chat, game-time tracking and more - but it’s up to you which features you want to use.

For Steam CD Keys, you must download (HERE), install the Steam software and create a new account. If you already have a Steam account, once you log in, click "Add a game" (located in the bottom left corner), and then "Activate a Product on Steam". After a few more clicks, you will be prompted to type in your CD Key. Thus, the game will be activated and automatically added to your account library.