I don't know a technical way to explain them, only my observations from using the programs. They are a bunch of pixels in millions of colors that the fractal program helps you to push around until you get an abstract shape and color that pleases you. You will find yourself playing with them for hours, and occasionaly one is gorgeous enough to actually SAVE. Sometimes you will save only a little corner of one, that suits your further purposes for screensavers.
I have done a "Fractal Cats" screensaver that gives my kittycats a totally unreal background for the real cats. Check out the free sample if you would like to see how they combine for a fascinating 'saver.
Fractals are beautiful on their own, or a super fanciful background for more ordinary objects like the cats. They would make for some interesting fantasy worlds as well.
Enjoy making some today!