title: difference between a field and the field.keyword
body: <p>If I add a document with several fields to an Elasticsearch index, when I view it in Kibana, I get each time the same field twice. One of them will be called</p>
  
  <pre><code>some_field
  </code></pre>
  
  <p>and the other one will be called </p>
  
  <pre><code>some_field.keyword
  </code></pre>
  
  <p>Where does this behaviour come from and what is the difference between both of them?</p>
  
  <p>PS: one of them is aggregatable (not sure what that means) and the other (without keyword) is not.</p>