1. A Microsoft Windows concept, a GUID is a large, unique number. Each Windows computer has its own GUID to identify it unique. Likewise, every time a user account is created, a GUID is assigned to the user. Key point: Microsoft got in trouble in 1999 for automatically shipping up the GUIDs as part of its registration process. Key point: GUIDs are sometimes placed within hidden fields within Microsoft documents (Word, Excel), allowing the original author to be identified. This was used in order to help track down the author of the "Melissa" worm. Key point: The GUID is generated from other "unique" information on a machine, such as the MAC address of the Ethernet adapter. It isn't guaranteed to be absolutely unique, but chances of any two GUIDs in the world being identical are remote. From Hacking-Lexicon