Battle.net is Blizzard Entertainment's game management system. It is used to activate, download, and play games such as Diablo, World of Warcraft, Starcraft and Overwatch. The publisher Activision has decided to use Blizzard's network for their latest games, including Destiny 2 and Call of Duty: Black Ops 4 via the Battle.net platform.
Battle.net also has a desktop client that you need to use for your games. Proceed as follows to use access your games:
Activations at Battle.net can be made conveniently via the internet browser. You can either use the predefined link from your Gamesplanet account (below the activation key) or you can visit the activation page to enter your activation key manually.
Once activated successfully, the game can be downloaded and played through the Desktop Client.