title: ArrayCollection in Symfony
body: <p>since I'm quite new to Symfony and Doctrine I got a maybe stupid question ;-)</p>
  
  <p>Can someone use simple words to explain Collections (especially ArrayCollections in entities) to me? What is it and when and how to use them? (Maybe in an simple example)</p>
  
  <p>Couldn't figure it out quite well in the docs...</p>
  
  <p>Thanks in advance.</p>