Well, a simle way of explaining what a emulator is, it's a program where you play games on. Visual Boy Advanced plays Game Boy Advanced and Game Boy Color games. You can add Gameshark Codes, Gamegenie Codes, Codebreaker Codes, and Action Replay Codes. You can also make your own hack, just like people made pokemon brown, pokemon aqua and pokemon gray. Right now I have the game pokemon brown. You can adjust image, speed and even screen capture so you can have your own image.
EDIT: There's also tools like map viewer and memory dumper for developers.
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