Musical Instrument Digital Interface
Musical Instrument Digital Interface, better known as simply MIDI is a simple networking protocol specifically designed for keyboard based musical instruments, rather than sending out audio data, it sends out standardised messages containing note, duration, volume information and so on.
Usually known as just GM, it is a standard for sound cards or modules that have a built in sample playback capabilities. It dictates that the receiving module has a specific type of sound at a specific address allowing software developers to write music or music program that target more than one specific device.
eXtended General MIDI
Also known as XG or "Yamaha XG", an extension to General MIDI that expands the number of available sounds to over 600, up from the 128 that the GM.
Similar to Yamaha's XG, it adds to the number of sounds available from the GM module.