Endianness is the term used to describe the byte order of data.
Big-endian means data is being sent or stored with the Most Significant Byte (MSB) first. Little-endian means data is sent with the Least Significant Byte (LSB) first.
A CPU always has an endianness, even for 8 bit CPUs. Any CPU stores data in units larger than just single bytes. The stored value can not also be looked at on a byte-by-byte basis. If the first byte is the LSB, then the CPU is said to be little-endian. If the first byte is the high order byte, it is said to be big-endian.
Example: Let's look at a 32-bit CPU, storing the value 0x12345678 in memory: (memory shown in hex) Big endian: 12 34 56 78
Little endian: 78 56 34 12
For most CPUs, the endianness is fixed by design. For those CPUs, Little-endian is more popular. All x86 CPUs use little endian. On ARM processors, the endianness is selectable by the chip designer, but in reality again a vast majority of CPUs is little endian, making it the de-facto standard. Motorola processors such as Power-PC are big-endian.
In the internet, the byte order is big-endian. This means that little endian processors are a slight disadvantagem as they need to swap endianness each time they write to a packet to be send or read from a received packet and kind of protocol information.