From Halopedia, the Halo wiki
Steam is an online platform that allows users to download and play PC games. The service is operated by Valve. Several Halo games on Steam offer Steam rewards which include Halo themed: trading cards, badges, emoticons, and backgrounds. Emoticons, and backgrounds can also be unlocked via the Steam Points Shop, which uses Steam Points, a currency earned by purchasing games, DLC, micro-transactions, and subscriptions.
Halo products available on Steam