A Globally Unique Identifier or GUID is a pseudo-random number used in software applications. While each generated GUID is not guaranteed to be unique, the total number of unique keys (2^128 or 3.4028×10^38) is so large that the possibility of the same number being generated twice is very small.
GUIDs are used in many pieces of software, including Oracle Database and Novell eDirectory, but the most high-profile GUID implementation may be Microsoft's. Theirs is an implementation of a standard called Universally Unique Identifier (UUID), specified by the Open Software Foundation (OSF).
A GUID is created by the computer's MAC Address and the timestamp of the current time/date.