At its core, Steam is a vast digital game store. Though you can access many of Steam's features in-browser, you can also download and install Steam as a standalone client for your desktop if you want to get the most out of the software.
Available for free online, Steam can run in your browser window. Steam has since moved beyond a simple game store to become a one-stop hub for PC gamers.
Though most game developers now have their own online storefronts, Steam was the platform that started it all.
