A screen saver (which is sometimes spelled erroneously without a space in between) is an animation that an operating system run when the computer is idle.
Screen savers were originally designed with one purpose in mind, it saves the screen, literally. Computer screens display images by constantly bombarding the screen with laser beams. One of the problems of early monitors is that, for graphics-intensive applications, static portions of the screen may result in ghosts, or afterimages that remain on the screen long after the image has been cleared. The reason for this is that the same image is beamed on the screen, and after a while, the image just becomes stuck. For Windows, the Start Menu and taskbar were immediate suspects, as well as the Office toolbars. The solution to this problem is a periodic display of animation to constantly shift the image displayed onscreen, and screen savers were born.
Many years later, monitor manufacturers have already addressed the ghost problem, thereby making screen savers absolutely useless, and yet the screen saver lingered, mostly because they are pretty to look at.
In a recent study, it has been found out that the majority of malwares propagate through popups offering free screen saver downloads. Avoid problems by downloading from trusted sites such as Microsoft or WinCustomize.