Constants

These constants are defined for use in Joomla and extensions:-

JPATH_ADMINISTRATOR	The path to the administrator folder.
JPATH_BASE	The path to the installed Joomla! site.
JPATH_CACHE	The path to the cache folder.
JPATH_COMPONENT	The path to the current component being executed.
JPATH_COMPONENT_ADMINISTRATOR	The path to the administration folder of the current component being executed.
JPATH_COMPONENT_SITE	The path to the site folder of the current component being executed.
JPATH_CONFIGURATION	The path to folder containing the configuration.php file.
JPATH_INSTALLATION	The path to the installation folder.
JPATH_LIBRARIES	The path to the libraries folder.
JPATH_PLUGINS	The path to the plugins folder.
JPATH_ROOT	The path to the installed Joomla! site.
JPATH_SITE	The path to the installed Joomla! site.
JPATH_THEMES	The path to the templates folder.
JPATH_XMLRPC	The path to the XML-RPC Web service folder.(1.5 only)

These constants are defined in _path_/includes/defines.php except JPATH_BASE which is defined in _path_/index.php.

Note: These paths are the absolute paths of these locations within the file system, NOT the path you'd use in a URL.

For URL paths, try using JURI::base().

Difference between JPATH_SITE, JPATH_ROOT, and JPATH_BASE

JPATH_SITE is meant to represent the root path of the JSite application, just as JPATH_ADMINISTRATOR is mean to represent the root path of the JAdministrator application.

JPATH_BASE is the root path for the current requested application.... so if you are in the administrator application:

JPATH_BASE == JPATH_ADMINISTRATOR

If you are in the site application:

JPATH_BASE == JPATH_SITE

If you are in the installation application:

JPATH_BASE == JPATH_INSTALLATION.

JPATH_ROOT is the root path for the Joomla install and does not depend upon any application.