Organizing content in Drupal can be very liberating — or very frustrating, depending on what methods you're used to using.
Almost all information in Drupal is stored as a 'node', the basic unit of content. By default, there is no hierarchy or structure imposed on these nodes: they do not reside in specific 'sections' of your site, and different kinds of nodes (images, blog posts, news articles, etc.) are not automatically grouped or sorted into different groups. Rather than hard-coding specific hierarchies or styles of display, Drupal treats your content as a giant soup, with each piece of content having properties like a title and an author, a 'published' flag, a publication date, and so on. Specific kinds of pages, specific sections of your site, and so on are created by pulling up any content with certain properties and listing it.
Some examples can help clarify this concept.
- Default front page