title: What exactly is a device in TensorFlow?
body: <p>I would be extremely helpful to have a clear definition what a device in TensorFlow is exactly. Is a device a single processing unit (no "real" concurrency possible)? </p>
  
  <p>You can define as many devices as you want by doing the following:</p>
  
  <pre><code>config = tf.ConfigProto(device_count={"CPU": 2},
                      inter_op_parallelism_threads=2,
                      intra_op_parallelism_threads=1)
  sess = tf.Session(config=config)
  </code></pre>
  
  <p>How is it possible that you can define as many devices as you want despite having only one processor with 4 cores?</p>