GUID is a globally unique identifier. It seems to be used interchangeably with UUID which stands for universally unique identifier. The GUID is a 128-bit quantity that identifies things in the Microsoft universe. A GUID, while treated as an atomic 128-bit unit, has the following partitions:
32 bits 16 bits 8 bits 8 bits 8 bytes
In multimedia terms, GUIDs figure prominently in Microsoft's ASF format.