The Universally Unique Identifier can be used to identify a device independent form its mount point or device name. This is more and more important as many devices today support hot-plugging or are external anyway. Therefore it makes sometimes sense to access a device (for example in fstab) not by device name but by the UUID.
There are several ways to get the UUID. The first one uses the /dev/ directory. While you are on is you might want to check other by-* directories.
1 $ ls -l /dev/disk/by-uuid
2 lrwxrwxrwx 1 root root 10 11. Okt 18:02 53cdad3b-4b01-4a6c-a099-be1cdf1acf6d -> ../../sda2
Another way to get the uuid by usage of the tool blkid:
1 $ blkid /dev/sda1
2 /dev/sda1: LABEL="/" UUID="ee7cf0a0-1922-401b-a1ae-6ec9261484c0" SEC_TYPE="ext2" TYPE="ext3"
There you also get the label and other information. Quite usefule.
Btw., if you wonder how "unique" this unique is, here a quote from Wikipedia:
1 trillion UUIDs would have to be created every nanosecond for 10 billion years to exhaust the number of UUIDs.