hyperreal's Nook of Apocrypha

[1] Headered means the first 16 bytes of the ROM contain information that tells the emulator how big the ROM is, the region the game is for, what mapper it uses, what type of mirroring it has, whether it is from a home console or system console. Most, if not all, emulators won't let you run the game if it doesn't have a header, so if you want to emulate these games, use these ROMs.

[2] Headerless ROMs cannot be played by emulators because the emulators won't know what to emulate unless a 16-byte header exists that tells them. Headerless ROMs exist for archival purposes only; they try to aim for a 1:1 copy of the catridge and can be used to create headered ROMs.

[3] Endianness describes the order in which data larger than one byte is stored. In big-endian order, the individual bytes that comprise a larger value are stored starting with the most significant byte. For example, if we have 1,000,000 stored in a 32-bit data type as a sequence of four bytes represented in hexadecimal, the big-endian order would be 0x000F4240. Since it is a 32-bit data type, and since there are 8 bits in one byte, the sequence can be separated into four bytes: 0x00 0x0F 0x42 0x40 = 0x000F4240. The most significant byte is 0x00, and the least significant byte is 0x40. In little-endian order, the individual bytes are stored starting with the least significant byte. So 1,000,000 stored in little-endian order would be 0x40 0x42 0x0F 0x00 = 0x40420F00. Big-endian is Nintendo 64's native order. Little-endian is x86's native order. Byte-swapped swaps bytes pair-wise from the native format. The native format is big-endian, so we take the big-endian format and swap the first byte (0x00) with the second byte (0x0F), and then swap the third byte (0x42) with the fourth byte (0x40). So 0x000F4240 (native/big-endian) becomes 0x0F 0x00 0x40 0x42 = 0x0F004042 (byte swapped). The format you select to download and run in an emulator will depend on which format the emulator supports. Many emulators support both big-endian and byte-swapped. The file extensions for big-endian, little-endian, and byte-swapped are, respectively, .z64, .n64, and .v64.