title: What is the difference between save a pandas dataframe to pickle and to csv?
body: <p>I am learning python pandas.
  I see a tutorial which shows two ways to save a pandas dataframe.</p>
  
  <ol>
  <li><p><code>pd.to_csv('sub.csv')</code> and to open <code>pd.read_csv('sub.csv')</code></p></li>
  <li><p><code>pd.to_pickle('sub.pkl')</code> and to open <code>pd.read_pickle('sub.pkl')</code></p></li>
  </ol>
  
  <p>The tutorial says <code>to_pickle</code> is to save the dataframe to disk. I am confused about this. Because when I use <code>to_csv</code>, I did see a csv file appears in the folder, which I assume is also save to disk right?</p>
  
  <p>In general, why we want to save a dataframe using <code>to_pickle</code> rather than save it to csv or txt or other format?</p>