Short answers, pulled from the story.
Steam was launched out of beta on the 12th of September 2003. It was originally designed as a software client to push game updates automatically for Valve's own games, solving the problem of multiplayer players disconnecting for several days while waiting to install patches.
Steam held an estimated 75% of the PC game distribution market as of 2013. By 2017, game purchases through Steam totaled about 4.3 billion, representing at least 18% of global PC game sales according to Steam Spy.
Steamworks is a free application programming interface released in May 2008 that allows game developers to integrate Steam's features, including digital rights management, matchmaking, cloud saving, and anti-cheat tools, into their games without Valve's direct involvement.
Valve launched Steam Direct on the 13th of June 2017, the week after shutting down Greenlight. Under Steam Direct, developers pay a recoupable $100 fee per game, complete identification and tax forms, and wait thirty days for Valve to review the submission. Valve refunds the fee once a game exceeds $1,000 in sales.
By 2021, Steam had over 132 million monthly active users. By 2018, the service had over 90 million monthly active users.
Steam was ported to macOS, with the client announced in March 2010 and launched on the 12th of May 2010. A Linux client was officially released in mid-February 2013. A mobile version for iOS and Android was released in 2012.