The following user guide provides a good description of Drupal's organization and function.
 
Drupal User Guide

Drupal stores content, user data, permissions, menus, blocks, views, comments and other configuration settings in a database.  In Drupal content is stored in a data structure known as a "node".  Drupal assigns each node with node id (abbreviated as $nid). Generally each node also has a title, an author, date last saved, a menu link, a description, a revision history, an alias, promotion setting and other meta data.  This allows Drupal to query the database and to display different "Views"  of nodes by title and other data tags.  

Drupal manages users by their userid (abbreviated uid)  User1 is the administrator of the Drupal site.  This user is somewhat comparable to a "root" user in Linux and has "super" powers over administrating the site. Drupal uses roles and permissions to assign privileges to users.  Users can be assigned different roles, e.g, Editor, Administrator, or custom created roles with associated user permissions.

Drupal separates data content from presentation. Site administrators can choose from a selection of Drupal "Themes" to change the look and organization of the site's display.  There are hundreds (or more) themes that can be downloaded and installed into a Drupal website.  It is also possible to modify or create new themes.     

Each Drupal site typically has a main theme for displaying Drupal pages to users and an "Administrative" theme for displaying site configuration and other data to users who have "Administrative" privileges.

The default Drupal admin menu contains the following menu tabs:

Content - Create, Edit, Delete
Structure- Create Blocks, Menus
Appearance - Select or add Themes
Extend -  Add or manage core or contributed modules
Configuration - manage configuration of the site and its components
People - Manage users, roles and permissions
Reports
Help