From [[http://nocash.emubase.de/everynes.htm]]:

===== Cartridge ROM-Image File Formats =====

==== iNES Format (.NES) ====

This fileformat and mapper-numbers have been designed/assigned by Marat Fayzullin (author of iNES emulator), please contact him if you want to make any changes to the format or numbers. The file header is 16 bytes:
  00h  File ID ('NES',1Ah)
  04h  Number of 16K PRG-ROM pages
  05h  Number of 8K CHR-ROM pages (00h=None / VRAM)
  06h  Cartridge Type LSB
   Bit7-4  Mapper Number (lower 4bits)
   Bit3    1=Four-screen VRAM layout
   Bit2    1=512-byte trainer/patch at 7000h-71FFh
   Bit1    1=Battery-backed SRAM at 6000h-7FFFh, set only if battery-backed
   Bit0    0=Horizontal mirroring, 1=Vertical mirroring
  07h  Cartridge Type MSB (ignore this and further bytes if Byte 0Fh nonzero)
   Bit7-4  Mapper Number (upper 4bits)
   Bit3-2  Reserved (zero)
   Bit1    1=PC10 game (arcade machine with additional 8K Z80-ROM) (*)
   Bit0    1=VS Unisystem game (arcade machine with different palette)
  08h  Number of 8K RAM (SRAM?) pages (usually 00h=None-or-not-specified)
  09h  Reserved (zero)
  0Ah  Reserved (zero) (sometimes 03h,10h,13h,30h,33h purpose unknown) (*)
  0Bh  Reserved (zero)
  0Ch  Reserved (zero)
  0Dh  Reserved (zero)
  0Eh  Reserved (zero)
  0Fh  Nonzero if [07h..0Fh]=GARBAGE, if so, assume [07h..0Fh]=ALL ZERO (*)

 
Followed by 512 byte trainer (if any, see Byte 6, Bit 2, mainly FFE games).
Followed by N*16K PRG-ROM pages (see Byte 4).
Followed by N*8K CHR-ROM pages (if any, see Byte 5).
Followed by 8K Play Choice 10 Z80-ROM (if any, see Byte 7, Bit 1) (*).
Followed by 128 (or 127) bytes title at end of file (ASCII, zero-padded) (*).
Items marked as (*) are regulary used, but not offical part of the format.
Many PC10 files declare Z80-ROM as additional VROM bank (instead Byte7/Bit1).

==== .UNF - Universal NES Image File Format (UNIF) by Tennessee Carmel-Veilleux ====

A "newer" fileformat dated back to 2000, the relation between iNES mapper numbers and UNIF MAPR names is still undocumented, and of course nobody uses files with .UNF extension. Still, it's having one or two useful features, and may become more popular if somebody dares to fix the MAPR problem, and to rename it from .UNF to .NES extension.
File Header (32 bytes)
  00h-03h: "UNIF" tag identifier
  04h-07h: Revision number ("currently 4, for REV 7b, Revision 6 of UNIF" Huh!)
  08h-1Fh: Reserved for future usage

 
The header is followed by whatever chunks, all chunks are optional, and may or may not be included in the file, only the PRG0 one is obviously required. Software may skip any chunks which are uninteresting or unrecognized, each chunk formatted as such:
  00h-03h: Chunk ID string (4-letter ASCII, described below)
  04h-07h: Length of Data Block in bytes (excluding above ID and length entry)
  08h... : Data

 
MAPR - Board Name (aka Mapper) (ASCIZ, suggested max: 32 chars)
  This uses ASCIZ strings to describe the board names (instead of iNES
  mapper numbers), it's meant to be more specific than mapper numbers,
  for example, it's using different names for different MMC1-boards.
  http://www.parodius.com/~veilleux/boardtable.txt
  http://www.parodius.com/~veilleux/boardnames

 
PRG0..PRGF - Binary data of the PRG ROM
CHR0..CHRF - Binary data of the CHR ROM (aka VROM in general)
  Normally using only PRG0 (and CHR0, if VROM used).
  In rare cases, if the cart contains more than 1 PRG (or CHR) ROM chip,
  then PRG1-F and CHR1-F may be used for the additional chips.

 
TVCI - Television Standards Compatability Information (One Byte)
  00h  60Hz/NTSC (USA, Japan, etc.)
  01h  50Hz/PAL  (Germany, etc.)
  02h  Compatible with both 50Hz and 60Hz refresh rates

 
CTRL - Controllers used by the cartridge (currently only 1 Byte / 8bit)
  Bit0   Regular Joypad
  Bit1   Zapper
  Bit2   R.O.B
  Bit3   Arkanoid Controller (presumably Paddle)
  Bit4   Power Pad
  Bit5   Four-Score adapter (NES 4-player adapter) (Not Famicom adapter!)
  Bit6-7 Reserved

 
MIRR - Name Table Mirroring (1 Byte)
  00h  Two-Screen Horizontal Mirroring (Hard Wired)
  01h  Two-Screen Vertical Mirroring (Hard Wired)
  02h  Single-Screen BLK0 (Hard Wired)
  03h  Single-Screen BLK1 (Hard Wired)
  04h  Four-Screens of VRAM (Hard Wired)
  05h  Mirroring Controlled By Mapper Hardware

 
BATR - Battery installed on Board (1 dummy byte)
  Presence of this chunk means yes, absence means no.

 
NAME - Game Title, ASCIZ String
  Game Title

 
READ - Readme/Comments/Notes/Credits
  Probably some sort of ASCII text of unspecified formatting

 
VROR - Allow homebrewn games to over-write VROM (1 dummy byte)
  Presence of this chunk means yes, absence means no.

 
PCK0..PCKF - 32-bit CRCs for PRG0..PRGF blocks (4 bytes, each)
CCK0..CCKF - 32-bit CRCs for CHR0..CHRF blocks (4 bytes, each)
  Intended "to make sth sure on EPROMs" ;-) Checksum algorythm not specified.

 
DINF - Dumper information block (204 Bytes)
  100 bytes  ASCIZ name of the person who dumped the cart
  4   bytes  day, month, year-lsb, year-msb when cartridge was dumped
  100 bytes  ASCIZ agent "name of the ROM-dumping means used"

 
Note: All words and dwords in header/chunks stored LSB first.