An emulator is a file that can be used to simulate an electronics operating system. For example, you need an emulator or file that emulates or acts as a Gameboy or game boy advanced system which is required for you to play Gameboy original or color and game boy advanced (GBA) games. There are other emulators as well like for Nintendo game consoles: Nintendo entertainment, super Nintendo entertainment, Nintendo 64, Nintendo Gamecube, Nintendo DS, and the Nintendo WII. Ones for SEGA systems are: SEGA Saturn, master system, genesis, Megadrive, Dreamcast, SEGA CD 32x, and arcade style systems. There is an emulator available for all editions of the Atari arcade system. There is one for the Apple iPhone\itouch, one for both versions of Microsoft's xbox system. There are some for Sony computer entertainment: Sony Playstation 1, Sony Playstation 2, and Sony Playstation portable. there is one for the Amiga system as well. go to http://www.emuparadise.org/ to get an emulator for one of these systems. You also need a file called a ROM (Read-Only memory) file just like a computer game is a Rom file readable by your computer there is one for these video game system emulators as well. These files are in different formats. Be very CAREFUL as some of these files may not work. here is a guide of the ROM file formats: Gameboy advance ROM format is .gba, the other Gameboy rom file formats are .gbc for gameboy color, and .gb for Gameboy original. not some emulators can read mor than one rom or game file format. Example the Gameboy advance emulator can read Gameboy original(.gb), Gameboy color (.gbc), and Gameboy advance (.gba). The ROM file for Nintendo 64 games in .n64. and so on. You can also play old games online at their website too. All that's required is the Java plug-in for internet explorer. Note some emulators require the 3d acceleration and audio software: Directx 9.0 or higher.
I have sucessfully emulated a Windows 98 virtual machine within the Bochs emulator.