Hooks are specified, by the developer, in Actions and Filters. Here is a (hopefully) complete list of all existing Hooks within WordPress.

Because Hooks are required by Actions and Filter you may hear the phrase "Action Hooks" and "Filter Hooks" used from time to time.

In technical and strict terms: a Hook is an event, i.e. event as understood by Observer pattern, invoked by the do_action()or apply_filters() call that afterwards triggers all the action or filter functions, previously hooked to that event using add_action() or add_filter(), respectively.

Reference http://codex.wordpress.org/Glossary