Definition:
A GUID (global unique identifier) is a term used by Microsoft for a number that its programming generates to create a unique identity for an entity like a Word document. GUIDs are wide utilized in Microsoft products to identify interfaces, duplicate sets, records, and alternative objects. different types of objects have different types of GUIDs – for example, a Microsoft Access database uses a 16-byte field to determine a unique identifier for replication.
Further Reading:
Book: Patterns of Enterprise Application Architecture by Martin Fowler .