Journaling allows the filesystem to repair itself after an improper shutdown or other type of damages. Journaling keep record transactions which are to be performed, or which have been performed.
Journaling filesystems have two main advantages.
1) Journaling filesystems are faster to check during the boot process.
2) If a crash happened, using the journal (file system log) the metadata can restored faster and more efficiently.
The ext3 file system is a journaling file system. Other major journaling filesystems are ReiserFS, JFS (IBM) and XFS (Silicon Graphics)