Content Management
People visit websites for the content. Content creators need to publish words and images, using different styles and layouts to make a page. Drupal allows for easy content management with an interface to create and edit content.
Content Types and Fields
Drupal starts with Page and Article content types. Pages just have a body, and articles have an image and tags. For most sites, you will need to define additional content types which require special fields. For example, if you have a Book content type to store items in a library, it would have an author field, and ISBN. By specifying these in Drupal, it will configure the database to store it and provide a form to edit Books.
Menus
Drupal has many navigation features. Powerful menu tools let you organize content how you want.
Blocks
Blocks are individually rendered parts of the page that let you control the layout by rearranging them in regions.
Users, Roles, Permissions
Drupal is great for supporting multiple users. If they have different access to the system, they are set up with roles and permissions.
Views
Views lets you create lists of content, on it's own page or in a block. It let's you control how items are displayed, filtered, and sorted.
Themes
You can download or create themes for Drupal which change the look and feel.
Modules
You will often need to install modules to change the functionality of the site. For advanced projects you may need to write some custom code.
Help
There is a help link in the toolbar that provides information on the modules you have installed.
Search the web and drupal.org for what you're looking for, there is usually an answer out there.
Translations
The user interface as well as the content can be translated to support international websites.