What is the “link” element in ATOM-feeds?

Could someone please help me understand what the “link” tags are used for within an ATOM feed?

• Do they point to a physical resource, or just like an identifier?
• What is the difference between link URLs in the beginning and for each “entry” block?
• Is it compulsory to have this linkURL?

Any information regarding this would be much appreciated!

I have provided an example snippet of code below.

<?xml version="1.0"?>
<atom:feed>

  <link rel="self" href="http://publisher.example.com/happycats.xml" />
  <updated>2008-08-11T02:15:01Z</updated>

  <!-- Example of a full entry. -->
  <entry>
    <title>Heathcliff</title>
    <link href="http://publisher.example.com/happycat25.xml" />
    <id>http://publisher.example.com/happycat25.xml</id>
    <updated>2008-08-11T02:15:01Z</updated>
    <content>
      What a happy cat. Full content goes here.
    </content>
  </entry>