What to Backup

I have retrospect but am a little unsure which critical files to backup. I am currently backing up the user files. Are there any others that are critical? I really don't want to back up the entire system.
Thanks.

Laserdoc
Four Dell's and a new iMac

You only need to backup whatever you consider to be important. Normally this would include any pictures, documents, music, movies...etc.
Either way, what you backup is up to the individual.