System 18 was Sega's arcade platform in the early 1990s (so named because it was more advanced than their earlier System 16). Only a handful of games were ever released on this hardware.
A System 18 board is a JAMMA arcade board that has plugins for separate system 18 ROM boards (basically the System 18 board plugs in to the arcade game, and you plug a ROM board into the System 18 board). The main CPU was a MC68000 (at 12MHZ), while a simple Z80 was used for sound (along with a Ricoh RF5c68 chip, and a pair of YM3438s).
The System 18 platform ran games at a resolution of 320 x 224 (not bad for display on an arcade monitor), in a total of 4096 colors. This hardware was capable of showing 128 sprites on screen at once (with hardware sprite zooming), and graphics could use up to six different layers (4 tile layers, 1 text layer, and 1 sprite layer). That may not sound like much now, but that was hot stuff back in 1990.
Games that ran on they System 18 hardware
I have owned a couple of System 18 boardsets in my time. They are rather picky when it comes to the exact output of the power supply, and the quality of the monitor. You may experience problems with a System 18 board that you do not have with other JAMMA compatible boards. But, you can plug in ROM sets from other System 18 boards without changing out the entire board. This is very useful, as you can buy dead System 18 boards for almost nothing, then pull the little ROM board off and plug it into your good System 18 board, for a quick game change. (Chances are good that the ROM board will be fine on a "dead" System 18 board, if not, you can always make new ones with an eprom burner).