J3.x:Creating a Plugin for Joomla/10/en
From Joomla! Documentation
$this->params: the parameters set for this plugin by the administrator
$this->_name: the name of the plugin
$this->_type: the group (type) of the plugin
$this->db: the db object (since )
$this->app: the application object (since )
Tip To use
JPlugin tests if the property exists and is not private. If it is desired for the default objects to be used, create un-instantiated properties in the plugin class (i.e.
protected $db; protected $app; in the same area as
protected $autoloadLanguage = true;). The properties will not exist unless explicitly created.