Or this page for more detailed informationIf journaling is turned on for a disk, Mac OS X maintains a continuous record of changes to files on the disk. If your computer stops because of a power failure or some other issue, Mac OS X uses the journal to recover the hard disk to the last acceptable state before it stopped.
It is a personal choice, but if you have time to measure boot time both with journaling on and off and post here you won't really miss those extra 10 seconds it takes to boot because you aren't short of timeDisabling Journaling cut my boot time by 10 seconds, and the entire system seems much more responsive. When I turn it back on and restart, I add 10 seconds to my boot time, and moving files around and installing apps seems to take slightly longer.
Journaling keeps a "journal" of all your hard drive reads and writes. Basically if you modify a file it will keep details on the modification until the file is fully written. Also if you move a file it will keep track of how much of the file has been moved from the origin and how much has been moved to the destination.What does Journaling do anyway? Is it better to have it on or off?