Globally unique identifier

A globally unique identifier or GUID (pronounced /ˈɡuːɪd/ or /ˈɡwɪd/) is a special type of identifier used in software applications to provide a unique reference number. The value is represented as a 32 character hexadecimal string, such as {21EC2020-3AEA-1069-A2DD-08002B30309D} and usually stored as a 128 bit integer. The term GUID usually, but not always, refers to Microsoft's implementation of the Universally Unique Identifier (UUID) standard.

The primary purpose of the GUID is to have a totally unique number.

Ideally, a GUID will never be generated twice by any computer or group of computers in existence. The total number of unique keys (2128 or 3.4×1038 - in relation there are about 1.33×1050 atoms on earth) is so large that the probability of the same number being generated twice is extremely small, and certain techniques have been developed to help ensure that numbers are not duplicated.