A plugin can hook into the core of PWG by registering event handlers with PWG. An event handler is nothing more than a function that PWG will call.

register_event_handler('delete_user', my_action_on_delete_user);

In this case, 'delete_user' is the name of the event to which the plugin will hook. PWG will automatically call your function when a user is deleted.

PWG will trigger actions and events. The only difference between actions and events from a coding perspective is that you are required to return a value in the case of an event (this return value will be used later by PWG).

An example of action is 'delete_user' or 'delete_elements'. Your plugin can take any required actions when a user is deleted by registering an event handler for 'delete_user'. While handling actions you are not expected to directly return a value that will be used by PWG, but you can modify any global variable in order to change the general behaviour.

An example of event is 'render_user_comment_content'. PWG will give in input the raw content of a comment, you can modify it at your will (for example allow bbcode or add smilies) and return the value:

register_event_handler('render_user_comment_content', 'my_function');
function my_function($content)
{
  $content = str_replace(':)', 'http://example.com/icon_smile.gif', $content);
  return $content; //<-- NOTE: this is how you return the value to PWG 
}