Evan Abrams offers a look at both the After Effects Fractal Noise and Turbulent Noise Effects and makes some general notes on the similarities and differences between the two and explores some of the possibilities for these two effects.
Fractal Noise or Turbulent Noise? What’s the difference and when should I use each or either of those effects
Evan notes that Fractal Noise can be looped, and is a little more processor intensive than Turbulent noise, both however use Perlin Noise to create greyscale noise values. Turbulent Noise is described as the modern, higher-performance implementation of the Fractal Noise effect.
AERich’s AEPortal has the definitive round up of tutorials and techniques for using Fractal Noise which you can find here.