A Globally Unique Identifier (GUID) is a unique reference number used in software development to identify information without the need for a central authority. GUIDs are typically represented as a 128-bit value, which allows for an extremely large number of unique combinations, making it highly unlikely for two GUIDs to be the same. GUIDs are commonly used in databases, applications, and systems to ensure that each record or object can be distinctly identified. This is particularly useful in distributed systems where multiple users or processes may create entries simultaneously, preventing conflicts and ensuring data integrity.