Nagios monitoring framework uses plugins to provide a variety of monitor types. There are more than 50 plugins available from the official Nagios site for the absolute majority of typical monitoring needs; available plugins are enough to built most of required monitors. Plugins allow to track such resources as IP connections, SQL servers, high-level protocols such as Jabber, hardware state and so on.
Note that plugins should be compiled prior to their using, this is primary difference between user-provided scripts: plugins follow the given API and are in most cases much faster and less resource-consuming than user-provided scripts; the latter should be used only if the monitoring task has no simple solution based upon Nagios plugins.
Related terms:
Nagios configuration
Nagiosgraph
Nagios scripts
