Fractals have a pattern that we see again after zooming in.
The pattern can be:
• perfectly the same, like the Sierpinski triangle at right, which looks exactly the same when zoomed in by 2
• based on a formula, like the Mandelbrot Set below, which looks a bit different as it is zoomed in
• only general, such as a tree which has a trunk, limbs, branches then twigs
Many things in the real world are like fractals: trees, shorelines, broccoli, lungs, ocean surface, etc.
(Note: the name "fractal" comes from the more technical definition involving "fractional dimensions"!)