Instead of using the 'Core Downloader' in RetroArch itself, users have to download them the way they would download DLC for any Steam game. The version of RetroArch on Steam is mostly the same as the one available on the official website or through itch.io, except for the way users download cores - which are what let RetroArch emulate different systems.
This comes after a year of beta testing, and should make using RetroArch easier.
What just happened? RetroArch, a frontend that brings together emulators for a wide range of classic computers and game consoles, is now available to the public through Steam.