Sega Master System
The original Japanese console. Sega created a floppy disk drive for the Mark III but it was never released.
The American and European platform featured a new name and a redesigned case.
Master System Plus
A standard Sega Master System but with a built-in game.
Master System II
A shorter Sega Master System, which was the equivalent to the top-loader NES, except it was only released in Europe.
Music and Sound
Sega had licensed the Texas Instruments SN76489 chip and included it inside the VDP (Video Display Processor) Custom Chip of the Master System. Depending on the version the physical chip is either called 315-5124 (SMS1) or 315-5246 (SMS2). Aside from Graphics, the chip can output three pulse wave channels and a noise channel. Features include volume control for each channel as well as an Envelope Generator for each instrument.
The Japanese Model of the Master System also had an additional Yamaha YM2413 FM Synthesis chip that can work in 2 different modes. In the first it can play up six FM channels as well as five channels with predefined percussion instruments, and in the second mode it can play nine FM channels but no percussion.
This sound hardware can also be found in several other Sega Systems like the Game Gear which also uses the VDP, although modified to offer stereo sound, or the Mark III, which had a YM2413 based FM Sound Unit available as add-on hardware.
The Master System's sound chip can go from frequency $3FF to $00.