Explain Drupal Architecture

Answer

 Drupal is based on less publicized but still widely used architecture Presentation Abstraction Control, or PAC. The menu system acts as the Controller. It accepts input via a single source (HTTP GET and POST), routes requests to the appropriate helper functions, pulls data out of the Abstraction (nodes and, from Drupal 5 onwards, forms), and then pushes it through a filter to get a Presentation of it (the theme system). It even has multiple, parallel PAC agents in the form of blocks that push data out to a common canvas (page.tpl.php).