MAME stands for Multi Arcade Machine Emulator. As the name suggests, it is
a program that emulates the hardware (and low-level firmware) of a massive
variety of arcade machines.

MESS stands for Multi Emulator Super System. It is an emulator for the
hardware of many different games consoles (e.g. Sega, NES, SNES, etc.)
as well as many different old games-oriented home computers (e.g. C64,
C128, ZX80, etc.)

While the two programs are conceptually different, and have different
development teams, they share a lot of code and are distributed together.

Packages can be built with additional FLAVORs:

  x11:  built with x11 support (default).

  gl:   built with GL support.

  sdl:  built with SDL support.
        This FLAVOR requires the sdl package.