Nagios Core is the back end and free open source version of Nagios XI. They both have similar functions but there are some key differences:
Nagios XI has a clean, AJAX driven User Interface where Core must have it's files edited for the most part through the Terminal, with XI's wizards you can change and create most of the major things from the GUI.
Nagios XI includes built in RRD Graphing
Nagios XI uses "Dashboards" Which allow you to choose from a ever growing set of monitoring plugins to assist you in being notified of issues virtually the moment they happen. This is also good for high level overviews of everything going on.
For some more information and screen shots of the different UI's please visit: Http://www.nagios.com/
Our sales team can also assist you, and you can reach them at:
U.S.: 1-888-NAGIOS-1 (1-888-624-4671)