As far as I know, ext3 is simply ext2, with journaling.
My grasp of what journaling is is hazy at best, but from what I gather, Linux's filesystem uses a sort of RAM-caching for stuff that would ordinarily get written to disk. This makes it fast and efficient, but it also means that if your computer crashes (due to a power outage or something - since Linux never crashes
), some of the stuff that was supposedly written to disk never actually made it to disk, but was only in a cache in RAM.
The result is that some bad hard drive errors can pop up when you boot up again, and you may have to run fsck and other nasty stuff, and could even lose important data.
Journaling makes it so a crash won't cause any loss of data.
As I said, my grasp of it is hazy
You could probably do some google searching and turn up some more technical information.
Anyhow, in short, it's a good idea to use ext3 if your kernel supports it (which it most likely does). You can convert ext2 to ext3 really easily by typing:
tune2fs -j /dev/hdaX
(turns on journaling for hdaX)