title: Difference between composer prefer-dist and prefer-source?
body: <p>Looking at the help for PHP Composer's <code>install</code> command, I see the following two options</p>
  
  <pre><code>$ composer help install
  Options:
   --prefer-source            Forces installation from package sources when possible, including VCS information.
   --prefer-dist              Forces installation from package dist even for dev versions.
  </code></pre>
  
  <p>What's a "dist" installation?  I poked around the composer site and Google but there didn't seem to be anything that addressed this (So I assume it's something core and obvious to folks familiar with Composer — apologies for the newbie question)</p>
  
  <p>I'm assuming <code>--prefer-source</code> is where Composer will ask Packagist for the repository location, and then checkout/clone/export/etc. the project itself. </p>
  
  <p>If so, then where does <code>--prefer-dist</code> download from?  What does it download?</p>