Can someone explain to me what Plexus is http://plexus.codehaus.org/ ?
How is it related to Maven?
Plexus is a platform consisting of an Inversion-of-Control container along with a number of utility libraries, including the Classworlds class loader framework. Maven was built on it but, as khmarbaise points out, a large amount of work has gone into replacing it with Guice.
Plexus also provides a number of utility libraries which are heavily used by the core Maven plugins -- plexus-utils, plexus-archiver and more. ~/.m2/repository/org/codehaus/plexus/ will be fairly busy on any machine that's run Maven.