Emulation is the “reproduction of the function or action of a different computer, software system, etc.” Video game emulation is similar to running virtual machines of other OSes on one computer, but can be much more complicated as there are some architectures that are very hard to code due to a lack of documentation.
By re-creating a system to work on another platform, you need a lot more power than the original system to run it. As game system’s are getting more and more advanced, creating emulators for them is much more complicated.
There are a variety of emulators from very experimental to stable, depending on the console, difficulty, and developer base. Today, there is usually a stable emulator for every retro console, and many are also working on newer consoles.
For example, the Wii U is Nintendo’s 8th Generation console, designed to compete with the PS4 and Xbox One. Surprisingly, there is already an emulator in development to recreate such a complex system. It has a lot of bugs, but many games can still be played thanks to raw GPU power allowing for the system to be brute forced.
To start the emulator, all you need to do is download CEMU from http://cemu.info/ and then obtain a WUD/ISO file of the game you want to run. These files can be found on the internet, but you should only download it if you own a copy of the game.
After you open CEMU and load up the game, you can map the game to your keyboard or USB controller and play.
Here is Smash Bros. Brawl for Wii U running on a computer with Windows 10, i7-6700k, and an NVIDIA GTX 1070. It runs at a smooth 60 FPS and is very playable.
There are many other emulators, for almost any game console you can imagine. They take a lot of time to create, but they preserve the console for years to come. Even without original hardware, the games can be played in the future using one system that can run various emulators. As consumers, you can now experience consoles without any hardware, just by downloading from the internet and trying it out.