What is the difference between a page and a node? What is the actual difference between a node and a page in Drupal 7? Why there is separate page.tpl.php and node.tpl.php pages. How is the theming applied for the actual page. 

Any link for understanding all this would be greatly appreciated.
node.tpl.php


  Default theme implementation to display a node.


page.tpl.php


  Default theme implementation to display a single Drupal page.


see image for more detail.