$GLOBALS

Contains a reference to every variable which is currently available within the global scope of the script. The keys of this array are the names of the global variables. $GLOBALS has existed since PHP 3.

$_SERVER

Variables set by the web server or otherwise directly related to the execution environment of the current script. Analogous to the old $HTTP_SERVER_VARS array (which is still available, but deprecated).

$_GET

Variables provided to the script via HTTP GET

$_POST

Variables provided to the script via HTTP POST

$_COOKIE

Variables provided to the script via HTTP cookies.

$_FILES

Variables provided to the script via HTTP post file uploads.

$_ENV

Variables provided to the script via the environment. Analogous to the old $HTTP_ENV_VARS array (which is still available, but deprecated).

$_REQUEST

Variables provided to the script via the GET, POST, and COOKIE input mechanisms, and which therefore cannot be trusted.

$_SESSION

Variables which are currently registered to a script's session. Analogous to the old $HTTP_SESSION_VARS array

For more in depth information, please visit php.net (a really great PHP resource site).