a difference between 'Document' and 'Node' is described on Document database structure.
To sum up: 'Node' contains the basic information about a document (node ID, alias path, template ID, content tree related information, etc.). One Node object can be related to multiple 'Document' object. Document can be culture specific and contains localized content (document name, description, etc.). Every 'Document' has only one parent 'Node'. That is why it can be simply localized in the content tree.
'Page' is a cached object (see more about page cache on
Cache types) which contains node, document and page template data.