What are transition effects?
Transition effects are commonly used in slideshow screensavers to add a sense of animation to an otherwise static presentation of images. In a traditional slide show, images are displayed one after another, with nothing in the middle. Many screen savers change this by inserting a special effect between two images.
One classic example, used not only in slideshows but in films as well, is the "fade in" and "fade out" effect: one image slowly fades out, while another image simultaneously fades in until it fully replaces the other.
There are countless transition effects, from "dripping paint" and "jumbled blocks" to "rain drops" and "venetian blinds". Screensaver developers tend to make a selection, and often allow you to turn on/off individual effects.