We can certainly say that 'content' is any material that makes up the web page, be it Drupal-generated content, such as the banner and buttons, or user content, such as the text of a blog. Within Drupal, 'content' has more narrow parameters.
When you create a story in Drupal, it is stored in a database as a node, and is assigned a node ID (nid). Some would say that, with respect to Drupal, content is limited to objects (stories, and so on) that can receive comments created by users, and are assigned a node id. Others say that it is any object in Drupal that can be on a page. These technical discussions can cause your eyes to glaze over. It would seem that the latter definition makes the most sense; however, there is one additional factor that we need to consider, and that is the layout of the Drupal admin functions.
Drupal provides admin functions for creating and maintaining content, and these functions list only those objects that receive a node id. Other objects, such as Blocks, are created and maintained elsewhere.