Submitting Forms Without Reloading The Page: AJAX Implementation In WordPress
If you have ever wanted to send a form without reloading the page, provide a look-ahead search function that prompts the user with suggestions as they type, or auto-save documents, then what you need is AJAX (also known as XHR). A behind-the-scenes request is sent to the server, and returning data to your form. Whenever you see a loader animation after you have made some action on the page, it’s probably an AJAX request being submitted to the server.
Articles on Smashing Magazine — For Web Designers And Developers