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http://linux.org.mt/article/filesystems"Whenever a computer is switched off without a proper shutdown there is the possibility that data on the disk becomes corrupted - that is, some of the data will have been written while some has not, leaving files or even internal filesystem data in a "half-finished" state.
Whenever that happens the system goes through a routine to check the disk for errors - "fsck" in Linux and "scandisk" in Windows. This is time-consuming, especially on today's very large disks. This check is also forced once every so many boot-ups, to make sure everything is working properly.
Journaling filesystems get rid of these problems. Instead of writing modified files directly onto their area on the disk, the system maintains a "journal" on the disk which describes all the changes which must be made to disk. Then, a background process takes each journal entry, makes the change and marks it as completed. If the system is halted without a shutdown, any pending changes are performed when it is restarted and the system is ready to continue running in seconds. Incomplete entries in the journal are discarded. This guarantees consistency and removes the need for a long and complex filesystem check on bootup."
