A 128-bit value based on time and space that can be used to uniquely identify an item. Globally unique identifiers (GUIDs) are used in the Component Object Model (COM) to uniquely identify classes and interfaces so that naming conflicts will not occur.
A GUID is virtually guaranteed to be unique across all systems at any time. You can generate GUIDs using the console-based uuidgen utility or using the Microsoft Windows–based guidgen utility in Microsoft Visual C++.
In Windows 2000, each object, object class, or object attribute in Active Directory is assigned a unique GUID when it is created. The GUID of an entity in Active Directory never changes, even if the entity itself is renamed or moved to another location. The GUID acts as a kind of permanent name for the entity within the directory to ensure that it can be positively identified when needed.
Microsoft BackOffice products such as Microsoft Exchange Server and Microsoft SQL Server also use GUIDs to uniquely tag objects. For example, the information store in Exchange Server has a base GUID that is used to generate individual GUIDs for all messages, attachments, and folder contents kept in the store. If you restore the information store from a backup, you need to run the command isinteg -patch before restarting the information store to change the base GUID. Running this patch ensures that new objects created in the information store do not accidentally end up with GUIDs that are identical to those of objects already existing in the information store. This could cause inconsistencies in the information store database.