All content on a Drupal website is stored and treated as "nodes". A node can be one of a number of content types. A node is any piece of individual content, such as a page, poll, article, forum topic, or a blog entry.
Treating all content as nodes allow the flexibility to create new types of content. It also allows you to painlessly apply new features or changes to all content of one type. Comments are not stored as nodes but are always connected to one.
A node has several default fields. You can create custom fields and define your own content types. When you install modules, many will include additional fields (i.e. Postal addresses, images, files, URLs, geographic geofields) and content types for you to use on your site, administered in the "Content Types" area under Structure in Drupal 7.