The most basic form of content on a Drupal site is a node.
In Drupal, almost every bit of content is a node. Nodes can be pages, videos, blog posts or polls. While comments are not nodes, they are tied to, and reference a node.
Treating all content as nodes allows the flexibility of creating new types of content. It also allows you to painlessly apply new features or changes to all content.
While nodes are implemented as entities in Drupal 7, nodes should be used instead, unless specifically referring to any functionality of the Entity API module.