title: What is the meaning of "axis" attribute in a Pandas DataFrame?
body: <p>Taking the following example:</p>
  
  <pre><code>&gt;&gt;&gt; df1 = pd.DataFrame({"x":[1, 2, 3, 4, 5], 
                          "y":[3, 4, 5, 6, 7]}, 
                        index=['a', 'b', 'c', 'd', 'e'])
  
  &gt;&gt;&gt; df2 = pd.DataFrame({"y":[1, 3, 5, 7, 9], 
                          "z":[9, 8, 7, 6, 5]}, 
                        index=['b', 'c', 'd', 'e', 'f'])
  
  &gt;&gt;&gt; pd.concat([df1, df2], join='inner')
  </code></pre>
  
  <p>The output is:</p>
  
  <pre><code>   y
  a  3
  b  4
  c  5
  d  6
  e  7
  b  1
  c  3
  d  5
  e  7
  f  9
  </code></pre>
  
  <p>Since <code>axis=0</code> is the columns, I think tha <code>concat()</code> only considers <strong>columns</strong> that are found in both dataframes. But the acutal output considers <strong>rows</strong> that are found in both dataframes.</p>
  
  <p>What is the exactly meaning of <code>axis</code> parameter?</p>