What is an emulator and where can i get one to play pokemon games on my computer?

I want to play older generation pokemon games like red/blue/yellow ruby/sapphire/emerald diamond/pearl/emerald and heartgold/soulsilver|||its a software which copies consoles like gba or ps2 or n64 you can download the emulator and the games which are called roms at coolroms.com|||Pokemon emulator.com