You've seen it. Maybe it was on a plane, perhaps it had been in a friend's home, however you saw people playing Nintendo, Sega, as well as PlayStation games on their own computers. And yet, when you searched for those particular games in Steam, nothing comes up. What is this witchcraft?
It's by no means new, but you should not feel bad for not knowing about it. This isn't just mainstream cultural expertise, and can be somewhat confusing for novices. Here is how emulation functions, and how to set this up on your Windows PC.
To play old school console games in your own computer, you need two items: a emulator and a ROM.