Sure easy one here.
Emulators are basically "Digital Nintendo's" Nintendo was base on 8 bits which basically is nothing considering the massive amount of power a computer puts out.
Think of Emulators as a virtual console system.
You download your emulator which is a very small file.
Nester is what I've used.
Then you download the ROMS. Roms are just the game coded out to work with the emulator.
Do a google search for Nintendo Roms. A game like Original Mario Brothers is only like 80kbs where as today games are 4 gigs. You could probably download every nintendo game ever and keep it under 10 megabytes.
There are also Sega Genesis and Super nintendo and other emulators.
I think there is a PS2 and XBOX one too but those take a good bit of computer knowledge to get working.
Anything else my friend?