This page discusses some general concepts that will be useful as you begin to explore Drupal. For more details on these concepts you may follow the links to additional documentation.
A node is the generic term for a piece of content on your web site. Depending on the type of node, different fields will be attached, and this is known as a content type. For example, a basic Page content type has attached fields such as title and body fields. Other examples of content type are: Book pages for use in Books, Discussion topics in forums, Blog pages in blogs, and News articles.
The word "node" is not meant in the mathematical sense as part of a network.
An entity type is a useful abstraction to group together fields. Entity types are used to store and display data, which can be nodes (content), comments, taxonomy terms, user profiles, or something custom developed.
Read more about Entities in the Entity API documentation.