OS X Mountain Lion: What is a Mac OS Extended (Journaled) volume?
Mac OS Extended format is a hard disk format that increases the number of allocation blocks on the disk from previous disk formats used by the Mac OS. This format also allows more than 65,000 files to be stored on the hard disk. Mac OS Extended format optimizes the storage capacity of large hard disks by decreasing the minimum size of a single file.
A Mac OS Extended volume can be journaled, which means that the operating system keeps a continuous log (journal) of the changes made to the files on the volume. This log helps the operating system restore the volume to a usable state if a power failure or other problem interrupts the disk’s operation and damages files.