Both types of pages figure out what to show the user by using Variants. Variants are output handlers, and every page should have at least one. Most pages will simply have only one. Pages with multiple variants will choose one and only one Variant to display content to the user and will use the Selection Rules to figure out which Variant to display.
- Custom pages
- Custom pages are defined completely by the administrator. Their path, access control and visible menu characteristics are completely arbitrary.
- System pages
- System pages are defined by Drupal and Drupal modules. They primarily override pre-existing pages to provide different functionality. They often do not allow such features as access control in favor of what already exists, and they will usually 'fall back' to default Drupal behavior.