A fantastic overview of the feTurbulence SVG filter, including interactive examples of all the relevant attributes, how they can be combined, and some of the cool things you can (glitch buttons, squiggly text animations, and dynamically generating extremely impressive paper textures).
In other words, the feTurbulence filter primitive generates and renders Perlin noise. This kind of noise is useful in simulating several natural phenomena like clouds, fire and smoke, and in generating complex texture like marble or granite.