First, it's worth noting that 'HFS Journaled' isn't actually journaled. It uses a scheme called softupdates, which is very similar, but not quite the same thing.
What it means, though, is basically as follows. When the hard disk goes to write out any changes to the filesystem, it notes down what it's doing in a separate place, called the journal. In particular, it notes down creating a new file, copying a file, or deleting a file (moving a file is really just copying it to a new place and then deleting the original, so it's noted as that). This way, if the machine restarts, it can look in the journal to make sure that the filesystem is still OK, rather than having to run slow disk-repair programs. It's basically a faster way of making sure that the hard disk's filesystem can't be corrupted.
The journal does not contain the contents of any files. This has two important consequences. The good news is that because your actual data doesn't get copied anywhere, there is no privacy problem with journaling. Your data only goes where you save it, and nowhere else. The bad news is that individual files can still be corrupted from a bad restart, because the journal has no record of what was in the file; it only knows that there's supposed to be a file there.
Actually journaling all of the data written to disk is quite slow, which is another reason it isn't done. Journaling only the metadata (where the file is, how big it is, what its name is, and so on) is faster, but currently it still incurs a small performance hit. Some journaling systems can actually be faster than no journaling at all, depending on the way they're written, but OSX is not at this stage yet.
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