Meaning – The term hook, refers to the location in a computerprogram where instruction is inserted for invoking a particular function.
A hook is a functionality provided by software for users of that software to have their owncode called under certain circumstances. That code can augment or replace the current code.
A callback could be thought of as a specific kind of hook. By implementing callback functionality into a system, that system is letting you call some additional code after an action has completed. However, hooking (as a generic term) is not limited to callbacks.
One usage of hooks is in video game mod development. A game may not allow mod developers to extend base functionality, but hooks can be added by core mod library developers.
Example of usage – “The Drupal CMS provides developers with hooks that let them take additional action after a “content node” is created. If a developer doesn’t implement a hook, the node is created per normal.”