Basically what you'll want to backup is
- home directories
- system-wide setting files, if you've altered them (they're not stock ones)
- databases
- user/group account information
..and so on, you can continue the list. Most of the users' own data to be backed up is inside their home directory, and the rest is probably system-wide, so it depends. There's no need to back up every single file on the system, since that can be reinstalled easily. If you wanted a "full" backup, you had another drive where you copied an image of the other one (actually several drives and a lot of space for multiple images), but if you just want a file backup..well, system admin
must know what on the system is so precious it can't be replaced in case of a disaster - that's what you back up
EDIT: about the backup..there are many ways, probably even ready applications, but I have used scripts that
find for example to create a list of certain files, maybe add some "static places", then use an arciving tool like
tar or
cpio to create an archive, then compress it and after this transfer the package to another place. If you want, you can use checksums for the package too, to ensure it doesn't change. The transfer method isn't (in my opinion) the most important part, gathering the needed information is. Just copying files over to some other place is not a good idea, I prefer creating an archive of them since that's how the directory paths, permissions and even date stamps stay untouched.
