Database abstraction layerAllow the use of different database servers using the same code base.
Extra blogapi updates for 5.x
Extra system updates for 5.x
File interfaceCommon file handling functions.
Form generationFunctions to enable the processing and display of HTML forms.
FormattingFunctions to format numbers, strings, dates, etc.
Hook implementations for user-created content types.
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.
System updates from 4.7 to 5.0
System updates from 5.0 to x.x@todo Start this series of updates at 2000.
Themeable functionsFunctions that display HTML, and which can be customized by themes.