title: How to understand the term `tensor` in TensorFlow?
body: <p>I am new to TensorFlow. While I am reading the existing documentation, I found the term <code>tensor</code> really confusing. Because of it, I need to clarify the following questions:</p>
  
  <ol>
  <li>What is the relationship between <code>tensor</code> and <code>Variable</code>, <code>tensor</code><br>
  vs. <code>tf.constant</code>, 'tensor' vs. <code>tf.placeholder</code>?</li>
  <li>Are they all types of tensors?</li>
  </ol>