title: What is exactly sklearn.pipeline.Pipeline?
body: <p>I can't figure out how the <code>sklearn.pipeline.Pipeline</code> works exactly.</p>
  
  <p>There are a few explanation in the <a href="http://scikit-learn.org/stable/modules/generated/sklearn.pipeline.Pipeline.html" rel="noreferrer">doc</a>. For example what do they mean by:</p>
  
  <blockquote>
    <p>Pipeline of transforms with a final estimator.</p>
  </blockquote>
  
  <p>To make my question clearer, what are <code>steps</code>? How do they work?</p>
  
  <p><strong>Edit</strong></p>
  
  <p>Thanks to the answers I can make my question clearer:</p>
  
  <p>When I call pipeline and pass, as steps, two transformers and one estimator, e.g:</p>
  
  <pre><code>pipln = Pipeline([("trsfm1",transformer_1),
                    ("trsfm2",transformer_2),
                    ("estmtr",estimator)])
  </code></pre>
  
  <p>What happens when I call this?</p>
  
  <pre><code>pipln.fit()
  OR
  pipln.fit_transform()
  </code></pre>
  
  <p>I can't figure out how an estimator can be a transformer and how a transformer can be fitted.</p>