| Database abstraction layer | drupal/ includes/ database.inc | Allow the use of different database servers using the same code base. |
| File interface | drupal/ includes/ file.inc | Common file handling functions. |
| Form generation | drupal/ includes/ form.inc | Functions to enable output of HTML forms and form elements. |
| Formatting | drupal/ includes/ common.inc | Functions to format numbers, strings, dates, etc. |
| Hooks | drupal/ includes/ module.inc | Allow modules to interact with the Drupal core. |
| Input validation | drupal/ includes/ common.inc | Functions to validate user input. |
| Menu system | drupal/ includes/ menu.inc | Define the navigation menus, and route page requests to code based on URLs. |
| Node access rights | drupal/ modules/ node.module | The node access system determines who can do what to which nodes. |
| Search interface | drupal/ modules/ search.module | The Drupal search interface manages a global search mechanism. |
| Themeable functions | drupal/ includes/ theme.inc | Functions that display HTML, and which can be customized by themes. |
