[Fou-Lu]

$_POST when your request is post, $_GET when you're request is get.
In HTML world, $_POST when your form has an action="post", $_GET when its passed through a querystring.

$_REQUEST is another one of those "brainchild" things that zend did, like register_globals and magic_quotes. Newer versions of PHP can control these variables (at perdir level) in PHP 5.3+. The ultimate fallback is still to variables_order which is by default EGPCS, or ENV, GET, POST, COOKIE, and finally SERVER. This is clearly a big problem as now you cannot tell which method provided what AND it's overridden in the order from left to right.
At the very minimum $_REQUEST should be manually overwritten as a merge of $_POST and $_GET, where $_POST overrides $_GET. My suggestion is to never use it.