The basic idea of Perlin noise is to add layers (octaves) of noises.
Higher octaves are more precise, in that they have more points (higher frequency), but are (usually) less weighted.
PCG Wiki References
The following images on Flickr have been generated using Perlin Noise:
Perlin noise - Wikipedia article on Perlin noise.
Making Noise - Ken Perlin's Discussion of Perlin Noise.
Perlin Noise - Hugo Elias: An overview of Perlin noise.
Procedural 3D Content Generation - Dean Macri and Kim Pallister: An Intel publication on using Perlin noise. Notably about terrain mapping. (Only the first page isn't dead)