NameDescription
Database abstraction layerAllow the use of different database servers using the same code base.
File interfaceCommon file handling functions.
Form generationFunctions to enable output of HTML forms and form elements.
FormattingFunctions to format numbers, strings, dates, etc.
HooksAllow modules to interact with the Drupal core.
Input validationFunctions to validate user input.
Menu systemDefine the navigation menus, and route page requests to code based on URLs.
Node access rightsThe node access system determines who can do what to which nodes.
Search interfaceThe Drupal search interface manages a global search mechanism.
Themeable functionsFunctions that display HTML, and which can be customized by themes.

Other projects: