From MAME Team:
MAME stands for Multiple Arcade Machine Emulator is a reference to the inner workings of the emulated arcade machines. This is done both for educational purposes and for preservation purposes, in order to prevent many historical games from disappearing forever once the hardware they run on stops working. In order to preserve the games and demonstrate that the emulated behavior matches the original, you must also be able to actually play the games. To operate, the emulator requires images of the original ROMs, CDs, or hard disks from the arcade machines, which must be provided by the user.