To quote: The ext3 filesystem is a journaling extension to the standard ext2 filesystem on Linux.
What's interesting here is that it's basically just the unjournalled ext2 filesystem, but with journalling added on. Even with journalling, it's still ext2. That is, an ext3 filesystem is still an ext2 filesystem. For that matter, an ext2 filesystem is already basically an ext3 filesystem. The matter of how and where journalling actually comes in to play is really quite interesting.
This is a transcription of a talk given in 2000 by Dr. Stephen Tweedie.