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If that subject means anything to you, then you probably had a Commodore 64. I, thanks to the wonders of emulators am learning how to use a computer that I used to play games on at my friend’s house when I was in grade 4.
There are a whole slew of games that have become public domain since the C64 was released. Good for me 
I’m currently running an emulator on my Mac, but if I upgrade my palm then I’ll be able to run the same software on that, under a palm emulator for the commodore.
Simply, an emulator takes the code from another system and makes that code think it is running on the hardware that it was programmed to run on.