All content on a Drupal website is stored and treated as "nodes". A node is any piece of individual content (e.g., a page, article, forum topic, blog entry, etc.). Note, though, that comments are not stored as nodes but are always connected to a node.
The ability to create different “content types” is a way Drupal allows you to have different kinds of nodes for different purposes. For example, an “article” is one content type, a “book page” is another, and a “blog entry” yet another. You can also create new content types of your own.
Treating all content as nodes provides a great deal of flexibility that facilitates and simplifies creating new types of content. It also makes it easy to apply new features or changes to all content of a particular type.