Neither actually
AJAX (Asynchronous Javascript And XML) uses javascript to communicate with the server and enables it to update just part of a webpage.
Benefits are that the user gets a much more 'interactive' experience and because you are not reloading the whole page it is a lot quicker.
The ascynchronous bit means it goes on in the background and doesn't 'freeze' up the page whilst waiting for its data.
See
wikipedia for more information