title: Maven proxy vs mirror
body: <p>I am finding it difficult to understand the difference between a mirror and a proxy defined in the settings.xml. I read that whatever a defined mirror is a mirrrorOf, all http requests to those(mirrorOf) repositories are overridden by a corresponding http request to the mirror. What is a proxy then? This question is in the wake of troubleshooting the following error I receive while I try to create a new project using mvn archetype:generate.</p>
  
  <blockquote>
    <p>The plugin 'org.apache.maven.plugins:maven-archetype-plugin' does not
    exist or no valid version could be found</p>
  </blockquote>
  
  <p>Any help?</p>