Drupal has a reputation for having a steep learning curve, but I remain convinced that it's largely overstated. Nonetheless, it's true that right from the start the new Drupal user has to contend with documentation that uses unfamiliar, arguably non-intuitive jargon. That said, "What's a node in Drupal?" is a harder question to answer than one might think. Because the pieces of Drupal are usually inter-connected, an easier term to define, like "CCK", depends on the reader understanding what a "node" is.