Nodes are pieces of content that Drupal uses to build pages. Nodes can be pictures, stories, blog entries, etc. Node references can record node relational information.

A node has the following:

NID - the Node ID
Type - the node type, e.g. page, blog, book page, etc.
Title - the title
Content - the content
Author - the author
Version - the version
Date - the creation date
Comments - a node may have multiple comments
 

When a node is displayed on a page, by default, its layout is determined by node.tpl.php template.  A type specific template can be created, for example, node-blog.tpl.php for a node of with type "blog".  Using this technique, you can create a custom layout for each node type.

For a listing of all nodes go to /admin/content/node, where you can bulk publish and delete batches of  nodes.