JAMMA is the 56 pin arcade game wiring harness adopted by the Japanese Arcade Machine Manufacturers Association. This is basically just a wiring scheme that allows you to quickly swap game circuit boards without rewiring your game cabinet.
A game that is JAMMA can simply be plugged into any JAMMA wired cabinet, and it will work, without any extra wiring of any sort. The original JAMMA standard allowed for three buttons per player, and some manufacturers used a set of normally unused pins to add a fourth button (SNK did this with the NEO GEO system).
JAMMA+ however is not a standard. What JAMMA+ means is that the game requires an extra connector (or two or three or twelve), in addition to the regular JAMMA connector.
These extra connectors are not standardized among game manufacturers at all. Half the time different games from the same manufacturer will implement JAMMA+ differently.
The entire Street Fighter 2 (and up) series is JAMMA+, as is the Mortal Kombat series, almost all newer driving games, and all newer 4-player games (such as Teenage Mutant Ninja Turtles and The Simpsons). Pretty much any game newer than 1987 that uses more than 4 buttons, has more than 2 players, or has any controls other than joysticks will be lumped under the JAMMA+ category.
JAMMA+ pretty much defeats the entire plug and play portion of the JAMMA standard. You simply can't swap JAMMA+ game boards around like standard JAMMA boards. But luckily, most joystick equipped JAMMA+ cabinets are at least fully backwards compatible with standard JAMMA titles. That means you can plug your Final Fight board into a Street Fighter 2 cabinet and get full action, but you can't plug Street Fighter 2 into Final Fight.
Other popular arcade wiring schemes include System 16 and the Konami Standard.