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.
$ time foo