They are not simple graphics but are based on mathematical functions. These functions give complex geometric shapes. In other words, each small portion of a fractal, when magnified, can reproduce exactly a larger portion. Well, I hope that you are not too discouraged. It's not necessary to understand the mathematics under fractals to appreciate them.
Fractals are everywhere.
The shapes of leaves, trees, clouds or rivers are all fractals. Fractals are the rule and not the exception in the nature.