A UUID (universally unique identifier) is an arbitrary 128-bit identifier, and a standard set of guidelines for generating such an identifier. The goal is that each identifier will (with overwhelming probability) be unique, without need of a central authority.
Some of the bits in a UUID are not random, and are used to indicate the UUID version number, etc.
The standard way to display a UUID is to use hex encoding, with four hyphens added at particular positions. It typically looks something like this:
See also GUID, the Microsoft version of UUID.
Any UUID (or GUID) can be converted into a URI by inserting "urn:uuid:" at the beginning, and then followed by the standard form, with letters in lowercase. This allows a UUID to be represented anywhere a URI is allowed, such as the identification of a node in a RDF graph.