Steam is a digital distribution service and storefront developed by Valve. It was launched as a software client in September 2003 to provide video game updates automatically for Valve‘s games and expanded to distributing third-party titles in late 2005. Steam offers various features, such as game server matchmaking with Valve Anti-Cheat (VAC) measures, social networking, and game streaming services. The Steam client functions include update maintenance, cloud storage, and community features such as direct messaging, an in-game overlay, discussion forums, and a virtual collectable marketplace. The storefront also offers productivity software, game soundtracks, videos, and sells hardware made by Valve, such as the Valve Index and the Steam Deck.
Steamworks, an application programming interface (API) released in 2008, is used by developers to integrate Steam’s functions, including digital rights management (DRM), into their products. Several game publishers began distributing their products on Steam that year. Initially developed for Windows, Steam was ported to macOS and Linux in 2010 and 2013 respectively, while a mobile version of Steam for interacting with the service’s online features was released on iOS and Android in 2012.
The service is the largest digital distribution platform for PC games, with an estimated 75% of the market share in 2013 according to IHS Screen Digest. By 2017, game purchases through Steam totaled about US$4.3 billion, or at least 18% of global PC game sales according to Steam Spy. By 2021, the service had over 34,000 games with over 132 million monthly active users. Steam’s success has led to the development of the Steam Machine gaming PCs in 2015, including the SteamOS Linux distribution and Steam Controller; Steam Link devices for local game streaming; and in 2022, the handheld Steam Deck tailored for running Steam games.

Reference: Wikipedia