What is the difference between little-endian and big-endian byte order?

Endianness is the order in which the bytes of a multi-byte value are stored in memory: little-endian puts the least significant byte at the lowest address, big-endian the most significant.

* Little-endian (x86) stores the least-significant byte at the lowest address; big-endian (network order) stores the most-significant first. *

A 4-byte number has to be laid out byte-by-byte, and there are two sensible orders. For the value 0x12345678:

x86 and x86-64 are little-endian — the "little end" (least significant byte) comes first. This is exactly what you must remember when reading a memory dump in a debugger: bytes appear "backwards" relative to how you'd write the number.

When it bites you:

• Network protocols use big-endian ("network byte order"), so cross-machine data needs byte-swapping.
• Binary file formats and cross-platform data exchange must agree on endianness or numbers come out scrambled.

Memory trick: Little-endian → little (least significant) end stored first.

Go deeper:

• Endianness (Wikipedia) — the canonical article on byte ordering, with per-address diagrams.