The Nutch Plugin System provides a way to extend nutch functionality. A large part of the functionality of Nutch are provided by plugins: All of the parsing, indexing and searching that nutch does is actually accomplished by various plugins.
In writing a plugin, you're actually providing one or more extensions of the
existing extension-points (hooks).
The core Nutch extension-points are themselves defined in a plugin,
the nutch-extensionpoints
plugin.
Each extension-point defines an interface that must be implemented by the
extension. The core extension-points and extensions available in Nutch are
listed in the {@link org.apache.nutch.plugin.Pluggable} interface.