FCEUX is a Nintendo Entertainment System (NES), Famicom, Famicom Disk
System (FDS), and Dendy emulator.  It supports NTSC (USA/JPN), PAL
(European), and NTSC-PAL Hybrid modes.

The FCEUX concept is that of an "all in one" emulator that offers
accurate emulation and the best options for both casual play and a
variety of more advanced emulator functions.  For pro users, FCEUX
offers tools for debugging, rom-hacking, map making, Tool-assisted
movies, and Lua scripting

FCEUX is an evolution of the original FCE Ultra emulator.  Over time
FCE Ultra had separated into many distinct branches.  The concept behind
FCEUX is to merge elements from FCEU Ultra, FCEU rerecording, FCEUXD,
FCEUXDSP, FCEUXDSP CE, and FCEU-mm into a single branch of FCEU.

As the X implies, it is an all-encompassing version of the FCEU emulator
that provides the best of all worlds for the general player, the
ROM-hacking community, and the Tool-Assisted Speedrun Community.