GUID - technical definition

(Globally Unique IDentifier) An implementation of the universally unique ID (see UUID) that is computed by Windows and Windows applications. Using a pseudo-random 128-bit number, GUIDs are used to identify user accounts, documents, software, hardware, software interfaces, sessions, database keys and other items.

First developed to keep track of instances of COM objects, GUIDs were created by an algorithm that used the MAC address of the local Ethernet card. Microsoft later dropped the Ethernet address due to protests that documents could be traced back to a particular user's machine. Following are common Microsoft GUIDs.

     Type    Used For

     AppID   Applications
     CLSID   COM object classes
     IID     Interfaces
     MID     Machines
     OID     Objects
     SETID   Set of objects

Other GUIDs
Other types of identifiers are also called GUIDs; for example, Atom feeds and some RSS feeds contain unique text-based numbers for each blog post. See Atom, UUID and OID.