dotCMS is a content based CMS - not a page based CMS. This is a powerful and important concept. Content in dotCMS does not necessarily live on ANY site, page or channel. Rather, when you add or edit content in dotCMS, the content is stored in a central content repository. Pages that use that content are merely linked to it. This allows for content in dotCMS to be reused across different pages, sites and delivery channels such as JSON, XML or RSS feeds.
In dotCMS, content is defined by "Content Types". When building a site in dotCMS, CMS Administrators design Content Types that fit their specific use case or content needs. For example, a "News" Content Type, will have different fields (properties) than, say, a Content Type made for storing company Employees.
The easiest way to think about a Content Type is as a content database that you get to design, manage, permission, query and finally display. When an CMS Administrator is building a Content Type, they are designing both the "content database" and the input form that content managers will use to create and manage content. Here is an example of a "News" contentlet and its custom fields that were defined as the News Content Type was created.