What is a Service?
A service is any object that is loaded via $modx->getService. It can be a custom class provided by the user, or by MODX itself.
Once an object is loaded with getService, it is accessible via $modx->(servicename). So, for example:
$modx->getService('twitter','myTwitter','/path/to/twitter/model/',array( 'api_key' => 3212423, )); $modx->twitter->tweet('Success!');
What are the Default Included Services?
A list of the core-included MODX Services is as follows: