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