Endian and endianness (or "byte-order") describe how computers organize the bytes that make up numbers.
Each memory storage location has an index or address. Every byte can store an 8-bit number (i.e. between
0xff), so you must reserve more than one byte to store a larger number. By far the most common ordering of multiple bytes in one number is the little-endian, which is used on all Intel processors. Little-endian means storing bytes in order of least-to-most-significant (where the least significant byte takes the first or lowest address), comparable to a common European way of writing dates (e.g., 31 December 2050).
Naturally, big-endian is the opposite order, comparable to an ISO date (2050-12-31). Big-endian is also often called "network byte order", because Internet standards usually require data to be stored big-endian, starting at the standard UNIX socket level and going all the way up to standardized Web binary data structures. Also, older Mac computers using 68000-series and PowerPC microprocessors formerly used big-endian.
Examples with the number
0x12345678 (i.e. 305 419 896 in decimal):
0x78 0x56 0x34 0x12
0x12 0x34 0x56 0x78
- mixed-endian (historic and very rare):
0x34 0x12 0x78 0x56