nagios-common is just the common files (libraries and such) that
nagios requires.
If you install
nagios2 you'll probably notice that
nagios-common will be marked for installation, too (as a dependency).
To check this, if you're using Synaptic, select the
nagios2 package, and click the "Properties" button in the toolbar, and cycle over to the "Dependencies" tab; you'll probably notice
nagios-common is listed there.
If you're using Aptitude, highlight the
nagios2 package and press ENTER - you'll see a whole bunch of information about the package (to get out of the package info screen, press Q).
