If you already know about Ajax, and want to use it in a plugin you are developing, skip to AJAX in Plugins.
Because of its responsiveness, Ajax technology is being adopted by all sorts of websites -- and WordPress is no exception. Currently, the core of WordPress uses Ajax only in the administration screens. For instance, Ajax is used for instant updates when you are doing comment moderation, and when you are adding and deleting items from lists such as categories, blogroll, and posts; Ajax is also the technology behind the auto-save functionality on post and page editing screens. Several themes and plugins also use Ajax; for instance, some post rating plugins use Ajax to store the visitor's rating in the database and then display an updated average rating.