What is /.journal?

[mylar]

On Tue, 2005-12-27 at 13:04, Rick Stevens wrote:
> On Tue, 2005-12-27 at 20:04 +0300, A.Fadyushin at it-centre.ru wrote:
> > The /.journal file is created when the _existing_ EXT2 filesystem is
> > converted to EXT3 filesystem. That file contains the filesystem journal
> > used by EXT3. The file is only created during the conversion EXT2->EXT3
> > because it is impossible to create separate region on the existing EXT2
> > filesystem for the journal - therefore, the journal is placed in the
> > file /.journal (which is similar to regular files but is unremovable
> > under EXT3). When the EXT3 filesystem is created anew, the journal is
> > placed on disk separately from the space used for files' data, so there
> > is no need to reserve that space via special file entry.
> 
> Perfect answer, Alexey.  That is correct.  If you convert an ext2 to
> an ext3 filesystem, the system must put the journal somewhere, so it
> creates a ".journal" file in the root of the filesystem and puts the
> journal there.  A freshly created ext3 filesystem reserves space OUTSIDE
> the namespace for the journal, hence it doesn't show up in a directory
> listing.
> 
> It is important that you NOT delete the ".journal" file (permissions are
> set so it's difficult to do anyway), because if you do, you will cease
> to have an ext3 filesystem and you may even have issues mounting it
> since the ext3 module wants a journal and won't be able to find it.
> 

In short what  exactly does a journal do ? I know it has something to do
with enabling the filessytem to recover.

Also, sometimes when/if my system  with ext3 filesystems crashes when i
restart the system asks if I want to run "fsck". Is it a good idea to
run fsck or can I skip that step on an ext3 filesystem ?