What is an UUID
An universally unique identifier (UUID) is a 128-bit number used to identify information in computer systems. Sometimes also the term global unique indentifier (GUID) is used as well instead of UUID. As the name already says UUID's are globally unique. Furthermore they do not depend for their uniqueness on a central registration authority or coordination. The probability that an UUID is duplicated is so close to zero that it is considered to be zero. Information that is labeled with UUID even from independent parties can later be combined to a single dataset or database without having any conflict at all. There are different versions for the UUID's.
UUID version 1 is generated from a time and node id, which is usually simply the MAC address. Version 2 UUID's are generated from an identifier, which most likely is a group or an user id, from the time and a node id. Version 4 UUID's are generater using a random. Version 3 and 5 mehthods produce deterministic UUID's generated by hashing a namespace identifier and name.