JAMMA stands for - Japanese Arcade Machine Manufacturers Association
Jamma is basicaly a wiring standard based on 3 buttons and a 4 or 8 way joystick. It uses a common board edge connector, and common wiring.
Where Jamma and the MVS differ is in the wiring. The edge connector is the same, and the wiring is pretty close, but there are some differences:
1) Jamma is Mono, MVS is stereo
2) Jamma is based on 3 Buttons, MVS uses 4 buttons
3) MVS also adds a select button (2, 4&6 slots)
4) Some Jamma boards use -5V, MVS does not
You can view or download my .pdf file that shows the different pinouts here.
There is also Jamma Plus, which can use 6 buttons, but that is another story as it uses a second set of connectors for the additional buttons.