A UUID/GUID (Microsoft's name for it) is as one person put it, is a generated value whose likelihood of being duplicated by any other generator (or for that matter the same generator) is about the same as the probability of random atoms rushing together in empty space to form a small walnut. The number of potential UUIDs in fact, exceeds the number of atoms in the Universe.
This makes them ideal for generating identifiers for distributed systems that need to periodically sync up, since each node of the system can generate IDs in the near-100% assurance that no other node would have assigned the same ID to something else. UUIDs have been used to distinguish OLE/COM service providers, uniquely identify disk drives and partitions without regard to where they are plugged in, and for many other purposes.
They are, in effect, very long (128 bit) random numbers and generated by much the same means. Java, in fact, has a UUID generator, so it's easy to create your own UUIDs.
One thing to note, however, is that since they are almost totally random, they make for lousy database keys, since they're rather large to be used as simple hash values, and they have no inherent order that would help optimise sequential searching and retrieval.
Some people consider UUIDs and GUIDs to be the same thing, but that's not entirely true. If you look at a Windows GUID, you'll notice that the displayed version of the ID is broken into parts. Originally Microsoft was parceling out specific high-level ID components for the benefit of themselves and registered developers. So only part of the GUID was truly random. Sometimes, indeed, very little of it.
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