Brownian motion is a physical phenomena that Einstein used to demonstrate the existence of atoms. It can be modeled with a random walk, each instant a direction is chosen independent of previous steps. By correlating the step with previous steps, the shape of the outer boundary can be varied. This not-quite Brownian motion is called Fractional Brownian motion.
PCG Wiki References
Fractal Brownian motion - Wikipedia article on fractional Brownian motion.
A Real-Time Procedural Universe, Part One: Generating Planetary Bodies - by Sean O'Neil.