(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 objectsOther GUIDsOther 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 syndication format, UUID and OID.