Part 1 provides a general overview.
Part 2 of this manual explains the installation of the components and the administration of refdb. These tasks should be performed by a system administrator, as you'll need certain privileges that a regular user should not have.
Part 3 explains the general use of refdb for the regular user. This is the place to look if you just need to add or retrieve references or if you want to generate bibliographies.
Part 4 contains all the nasty details of the applications that refdb ships with: All these confusing command-line switches, interactive commands, and configuration file variables that your brain refuses to memorize.
Part 5 is meant for programmers who want to modify or extend the behaviour of refdb. You'll find a few internals which are of no interest for the regular user but might be of interest for people trying to customize or extend refdb.