| Actions | Functions that perform an action on a certain system object. |
| Ajax framework | Functions for Drupal's Ajax framework. |
| Ajax framework commands | Functions to create various Ajax commands. |
| Authorized operations | Functions to run operations with elevated privileges via authorize.php. |
| Batch operations | Creates and processes batch operations. |
| Code registry | The code registry engine. |
| Content markers | Markers used by theme_mark() and node_mark() to designate content. |
| Database abstraction layer | Allow the use of different database servers using the same code base. |
| Default theme implementations | Functions and templates for the user interface to be implemented by themes. |
| Field API | Attach custom data fields to Drupal entities. |
| Field API bulk data deletion | Clean up after Field API bulk deletion operations. |
| Field Attach API | Operate on Field API data attached to Drupal entities. |
| Field CRUD API | Create, update, and delete Field API fields, bundles, and instances. |
| Field Formatter API | Define Field API formatter types. |
| Field Info API | Obtain information about Field API configuration. |
| Field Language API | Handling of multilingual fields. |
| Field Storage API | Implement a storage engine for Field API data. |
| Field Types API | Define field types. |
| Field Widget API | Define Field API widget types. |
| File interface | Common file handling functions. |
| File module public API functions | These functions may be used to determine if and where a file is in use. |
| Form builder functions | Functions that build an abstract representation of a HTML form. |
| Form generation | Functions to enable the processing and display of HTML forms. |
| Formatting | Functions to format numbers, strings, dates, etc. |
| Hook implementations for user-created content types | Functions that implement hooks for user-created content types. |
| Hooks | Allow modules to interact with the Drupal core. |
| HTTP handling | Functions to properly handle HTTP responses. |
| Image toolkits | Functions for image file manipulations. |
| Input validation | Functions to validate user input. |
| Locking mechanisms | Functions to coordinate long-running operations across requests. |
| Logging severity levels | Logging severity levels as defined in RFC 3164. |
| Menu context types | Flags for use in the "context" attribute of menu router items. |
| Menu flags | Flags for use in the "type" attribute of menu items. |
| Menu item types | Definitions for various menu item types. |
| Menu status codes | Status codes for menu callbacks. |
| Menu system | Define the navigation menus, and route page requests to code based on URLs. |
| Menu tree parameters | Parameters for a menu tree. |
| Node access rights | The node access system determines who can do what to which nodes. |
| Node API Hooks | Functions to define and modify content types. |
| Pager pieces | Theme functions for customizing pager elements. |
| PHP wrapper functions | Functions that are wrappers or custom implementations of PHP functions. |
| Private update status cache system | Functions to manage the update status cache. |
| Queue operations | Queue items to allow later processing. |
| RDF Mapping API | Functions to describe entities and bundles in RDF. |
| Sanitization functions | Functions to sanitize values. |
| Schema API | API to handle database schemas. |
| Search interface | The Drupal search interface manages a global search mechanism. |
| Standard filters | Filters implemented by the filter.module. |
| Taxonomy indexing | Functions to maintain taxonomy indexing. |
| Translation import/export API. | Functions to import and export translations. |