In the configuration of a maven plugin's build, where you are specifying configuration for the "maven-plugin-plugin" there is something called an extractor. I also see it when building the plugin (Applying extractor for language: java).
In this page it specifies many different extractors, but doesn't give a very clear explanation of what they are?
An extractor in that meaning is responsible for extracting the description of the plugin like parameters, injectors etc. which is defined by javadoc things like or annotations or for ant like plugins:
@parameter
@goal
They exists currently for Java and BeanShell.