Drupal has many hooks, such as hook_menu and hook_form. What are some functions in Joomla analogous to these?
In Drupal, a "hook can be thought of as an event listener in the sense that an event triggers an action" (see https://www.drupal.org/node/292 for a more detailed explanation).
In Joomla, I think the closest equivalent would be a "plugin". Plugins are also triggered by events. You can find a list of plugin events at: https://docs.joomla.org/Plugin/Events