UUID is a way to specify exactly which physical drive and partition you want to use, instead of the traditional /dev/hd* block devices that the system assigns to them. It's often used in situations where the /dev entry corresponding to each drive can change, such as when hotplugging drives. It seems that Mandriva has switched to using UUID's by default.
You can also use the symlinks found in /dev/disk/by-uuid to do the same thing.
As for your next question, you certainly do want to replace "defaults" with the options you want. After all, things like umask and iocharset are not default options, are they?
When working with configuration files, you should always take the time to understand what the lines you're adding or modifying do. Try reading the man pages for mount and fstab for more details.
