Drupal main parts

Module

A module is software (code) that extends Drupal features and/or functionality. Core modules are those included with the main download of Drupal.

User, Permission, Role

Every visitor to site, whether they have an account and log in or visit the site anonymously, is considered a user to Drupal. Each user has a numeric user ID, and non-anonymous users also have a user name and an email address.

Node

A node in Drupal is the generic term for a piece of content on web site.

Also there are: Comment, Taxonomy, Database, Path, Theme, Region, Block, Menu.