| drupal-5/ modules/ system/ system.install | @todo Start this series of updates at 2000. |
| Database abstraction layer | drupal-5/ includes/ database.inc | Allow the use of different database servers using the same code base. |
| File interface | drupal-5/ includes/ file.inc | Common file handling functions. |
| Form generation | drupal-5/ includes/ form.inc | Functions to enable the processing and display of HTML forms. |
| Formatting | drupal-5/ includes/ common.inc | Functions to format numbers, strings, dates, etc. |
| Hook implementations for user-created content types. | drupal-5/ modules/ node/ node.module | |
| Hooks | drupal-5/ includes/ module.inc | Allow modules to interact with the Drupal core. |
| Input validation | drupal-5/ includes/ common.inc | Functions to validate user input. |
| Menu system | drupal-5/ includes/ menu.inc | Define the navigation menus, and route page requests to code based on URLs. |
| Node access rights | drupal-5/ modules/ node/ node.module | The node access system determines who can do what to which nodes. |
| Search interface | drupal-5/ modules/ search/ search.module | The Drupal search interface manages a global search mechanism. |
| Themeable functions | drupal-5/ includes/ theme.inc | Functions that display HTML, and which can be customized by themes. |
