Are those GIFs above familiar? Yes, they are what we call preloaders. They let the users know that a certain page or data was still loading or processing at the moment. Even a simple phrase, "Please wait a moment, processing data..." is called a preloader.
Usually, preloaders are used in registration validation, installers, searching and many more. The most common site I know who uses this is facebook. Look at the fourth GIF above, how many times have you seen that in facebook, huh?
But what's its used? Are they just some decorations on the site?
Yes, they are somewhat a decoration or something but a useful decoration. Imagine when a site is still processing a data and all it this is a white screen, you'll most likely think that the site has stop responding because all you can see is just plain white. So you'll end up refreshing the page. But using a preloader, you know that a certain site was still loading a data.
Copyright: I don't own the images used in this post