Last night, Oleg (litwol) said that Drupal is one function: module_invoke_all().
Funny, but it does reveal the essence of how contrib interacts with core.
Drupal is One Function
About Aaron Winborn
Aaron Winborn was Advomatic's first full time hire in 2006, and is a very active leader in the Drupal community. His first book, Drupal Multimedia is now available from Packt Publishing.