This article is from the Fractal FAQ, by Ermel Stepp stepp@muvms6.mu.wvnet.edu with numerous contributions by others.
The Mandelbrot set iterates z^2+c with z starting at 0 and varying c.
The Julia set iterates z^2+c for fixed c and varying starting z values. That
is, the Mandelbrot set is in parameter space (c-plane) while the Julia set is
in dynamical or variable space (z-plane).
 
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