As Keith mentioned, your photos (the files) are not actually 'in' the Lightroom catalog.
So there are two separate things you need to backup. One is the LR catalog file. This should be pretty automatic, you probably see it asking you (about once a week) if you want to back up the catalog.
The catalog file stores all the edits/changes that you've made to your photos within Lightroom. It also remembers what photos you've imported and where they are.
The second thing you need to back up, are the image files themselves. Mine are automatically backed up on an external drive with the rest of my important files.