You can archive the catalogs (into cab files) and then back these up (this archive can be done manually or via a commandline tool), or alternatively backup the data files that are on disk (ie within the data directory) but when doing this you must ensure that you back up the repository at the same time.  The second option means that to restore, you should really restore the repository and all catalogs (and cubes within), whereas archiving the catalogs individually and then backing these up allows you to restore individual catalogs rather than the whole server.  The operations guide talks about these options in more detail (with reference to Release Management but it's still a useful reference), you can find it here --> http://www.microsoft.com/technet/prodtechnol/sql/2000/maintain/anservog.mspx