There are two types of UUID's which uuidgen can generate: time-based UUID's and random-based UUID's. By default uuidgen will generate a random-based UUID if a high-quality random number generator is present. Otherwise, it will chose a time-based UUID. It is possible to force the generation of one of these two UUID types by using the -r or -t options.

-r

Generate a random-based UUID. This method creates a UUID consisting mostly of random bits. It requires that the operating system have a high quality random number generator, such as /dev/random.

-t

Generate a time-based UUID. This method creates a UUID based on the system clock plus the system's ethernet hardware address, if present.