$_GET returns an array of url 'parameters'. It is one way to access the 'query string'. See also $_POST and $_REQUEST.
There is no concept in PHP of url parameters automatically appearing in variables of the same name. A faint bell is ringing that maybe once there was something like this, I know some languages (I think Oracle ApEx) do this, but it can open up security risks and I think is generally considered to be A BAD THING.