In Maven, what is the difference between an extension and a plugin

Question

What is the functional difference between an extension and a plugin in Maven?

The below examples are taken from here which provides a brief summary but doesn't explain the difference well.

Extension example:

<project>
  ...
  <build>
    <extensions>
      <extension>
        <groupId>org.apache.maven.wagon</groupId>
        <artifactId>wagon-ftp</artifactId>
        <version>2.10</version>
      </extension>
    </extensions>
  </build>
  ...
</project>

Plugin example:

<project>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.felix</groupId>
        <artifactId>maven-bundle-plugin</artifactId>
        <extensions>true</extensions>
        <configuration>
          ...
        </configuration>
      </plugin>
    </plugins>
  </build>
  ...
</project>

Answer 1

From the docs:

Extensions are a list of artifacts that are to be used in this build. They will be included in the running build's classpath. They can enable extensions to the build process (such as add an ftp provider for the Wagon transport mechanism), as well as make plugins active which make changes to the build lifecycle. In short, extensions are artifacts that activated during build. The extensions do not have to actually do anything nor contain a Mojo. For this reason, extensions are excellent for specifying one out of multiple implementations of a common plugin interface.

The above are usually call build extensions which are often use the org.apache.maven.AbstractMavenLifecycleParticipant for particular usage.

A plugin which is defined with

<extension>true</extension>

will usually define it's own life cycle or packaging types like the maven-bundle-plugin which makes it possible to define a <packaging>bundle</packaging> or Maven Tycho defines packaging types: <packaging>eclipse-plugin</packaging>.