The NES file format, typically with the '.nes' extension, is a ROM image containing the program code and data for a Nintendo Entertainment System (NES) game. These files are essentially complete copies of the game cartridges, allowing users to play classic NES games on emulators. The NES format is relatively simple, consisting of a header followed by the program ROM and character ROM (CHR-ROM). The header contains information about the game, such as the mapper number, the size of the ROM and CHR-ROM, and whether the game uses vertical or horizontal mirroring. Emulators use this information to correctly load and run the game. Due to the age and popularity of the NES, a vast library of NES ROMs is available online, although their distribution is often subject to copyright laws. The NES format is a cornerstone of retro gaming and continues to be enjoyed by enthusiasts worldwide. The format's simplicity has also made it a popular target for homebrew game development.