A Globally Unique Identifier (in Microsoft terminology) or Universally Unique Identifier, is a 128-bit reference number. GUIDs / UUIDs are extremely unlikely to repeat, and therefore are assumed to be unique in programming applications.
You can use the tool below to generate them.
It is possible that an identifier could clash, however you would be very unlucky.
To have more than a 50% chance that a generated unique ID will collide with another 2,710,000,000,000,000,000 (2.71 Quintillion) IDs would have to be created. 43.36 Exabytes of storage would be required to store all of these combinations!
As of current (27th March 2021), there are 5 version of GUID. The specific details for each version and how to generate them can be found in this Internet Engineering Task Force document.
This tool is version 4 compliant.