Web definition - A Globally Unique Identifier or GUID is a pseudo-random number used in software applications. Each generated GUID is "statistically guaranteed" to be unique. This is based on the simple principle that the total number of unique keys ( or ) is so large that the possibility of the same number being generated twice is virtually zero.
Windows-specific definition - Short for Globally Unique Identifier, a unique 128-bit number that is produced by the Windows OS or by some Windows applications to identify a particular component, application, file, database entry, and/or user. For instance, a Web site may generate a GUID and assign it to a user's browser to record and track the session.
A GUID is also used in a Windows registry to identify COM DLLs. Knowing where to look in the registry and having the correct GUID yields a lot information about a COM object (i.e., information in the type library, its physical location, etc.). Windows also identifies user accounts by a username (computer/domain and username) and assigns it a GUID. Some database administrators even will use GUIDs as primary key values in databases.
GUIDs can be created in a number of ways, but usually they are a combination of a few unique settings based on specific point in time (e.g., an IP address, network MAC address, clock date/time, etc.).
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