system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Steam, the gaming software

I finally have a computer that can handle it.  
What is it?  How does it work?  (INB4 Miracles)
gpt
It's a store, game library, and social media program rolled into one. 

So you either buy a game through the built-in store or enter a serial key for a game that is in the list of available games (e.g if you bought a DVD copy from a store).  From there, Steam will download all the files for you  -- you can choose what to install or not; you can install/uninstall at will.  Once a game is in your library, it is permanently associated to your account.  If you buy a new computer or have more than one computer, you don't need any discs, just your steam account and an internet connection. 

This is coupled with a sort of social media aspect -- you get a profile page that lists some statistics like what you most recently played or how many hours you've put into various games.  There's also an instant messenger feature built in so you can keep a friend list and chat with them when they're online.  

It also tracks other information/statistics like which of your Steam friends have the same game (helps in finding multiplayer buddies) or what achievements you've garnered.