Events are used to plug into the map rendering process or events that happen as the map is interfaced with. Events can be defined for the map itself or for specific layers. For example:
$map_array = array(
  'events' => array(
    'beforeCenter' => array('customCallback'),
  )
  'layers' = array(
    'default_vector' => array(
      'events' => array(
        'moveend' => array('customModuleLoadSomeData','customModuleDoSomethingElse'),
        'zoomend' => array('customModuleLoadSomeData'),
      ),
    ),
  ),
);
Basically, on the PHP side, you are simply defining JS callbacks to be used.

Rendering Events

The following are custom rendering event points provided by the OpenLayers module:
beforeEverything
Executed as soon as possible for each map. The OpenLayers map object is not yet built.
beforeLayers
After the map object is built, but before layers are created and added.
beforeCenter
Before the map is zoomed and centered.
beforeControls
Before controls are added to the map
beforeEvents
Before events are added to the map
beforeBehaviors
Before behaviors are triggered
mapReady
Last call

OpenLayers Map Events

The OpenLayers map object has built-in events. See the supported map event types.

OpenLayers Layer Events

The OpenLayers layer object has built-in events. Here are a few links to some of the common layer types and events available to them: