Nagios is a program that will monitor hosts and services on your network. It has the ability to send email or page alerts when a problem arises and when a problem is resolved. Nagios is written in C and is designed to run under Linux (and some other *NIX variants) as a background process, intermittently running checks on various services that you specify.
The actual service checks are performed by separate "plugin" programs which return the status of the checks to Nagios. The plugins are available at http://sourceforge.net/projects/nagiosplug.
This package provides the core program, web interface, and documentation files for Nagios. Development files are built as a separate package.