Preloader is usually referred to the Gif image shown to user to indicate wait or processing state.
Preloader is usually referred to the Gif image in-which an indication for the user to wait or processing state. It seems like the following images samples:
Preloaders are widely used with ajax era because it usually includes some changes that may take some time without reloading the page so the user should be visually alerted that something is being done.
List of some popular online preloaders generators
