From LQWiki

Endianness is a concept that determines how the bytes of some integer data are to be stored in memory. There are conceivably several ways to do this, but the two main types are

• big endian - if we have the integer 0xfb04acd9, we would store it as fb 04 ac d9, with fb being stored at the lowest address and d9 at the highest address.
• little endian - if we have the same integer 0xfb04acd9, we would store it as d9 ac 04 fb

So called "middle endian" formats also exist (where the byte order may be in the form 04 fb d9 ac for 0xfb04acd9), but are much rarer.

Architectures

Most processors, including the IBM 370 family, the PDP-10, the Motorola microprocessor families, and most of the various RISC designs are big-endian. Big-endian byte order is also sometimes called network order, since data over a network is defined to be transferred in the big-endian schema.

MOS Technology 6502, Intel x86 and DEC VAX systems are little-endian.

Problems

Problems can arise when sending data between architectures, since, for example, big-endian data when extracted is presumed to be little-endian and is reconstructed incorrectly, or vice versa. This problem is sometimes known as the NUXI problem. Byte-swapping software, such as featured on the portable OPENSTEP operating system can alleviate this.