UUID’s are useful for making unique reference numbers of things. In an ideal world these UUIDs are so long, they never collide.
The first place to discuss UUIDs would be RFC 4122. The next part would be the wikipedia entry for a more concise version.
So why generate UUID’s? One reason could be to generate a unique code to identify a particular PC (mac address?). Another is to identify points in time (and can include the machine aka node’s mac address that created the UUID).
Java 6 api UUID entry
Blog that touches on UUID support in Java