GUID stands for Globally Unique Identifier.
GUID is the Microsoft implementation of the Distributed Computing Environment (DCE) Universally Unique Identifier(UUID).
A GUID is a 128-bit value consisting of one group of 8 hexadecimal digits, followed by three groups of 4 hexadecimal digits each, followed by one group of 12 hexadecimal digits.
The following example GUID shows the groupings of hexadecimal digits in a GUID:
According to Microsoft’s article GUID Structure :
There are four parts of GUID
Part 1: Specifies the first 8 hexadecimal digits of the GUID.
Part 2: Specifies the first group of 4 hexadecimal digits.
Part 3: Specifies the second group of 4 hexadecimal digits.
Part 4: Array of 8 bytes. The first 2 bytes contain the third group
of 4 hexadecimal digits. The remaining 6 bytes contain the
final 12 hexadecimal digits.
There are 2 ^ 128 possible GUIDs which is equivalent to approximately 40 Billion Billion Billion for every person on earth.