What exactly characterises Perlin noise? I've looked around online a little and found mainly explanations of its uses and a few bits of code for generating it (mainly through spectral synthesis), but from reading the code it's not obvious to me what the underlying principle is.

I know a little bit about the uses of noise in the musical domain, where there's a sweet spot in the parameters for noise generation that makes stochastically generated music sound "just random enough". It wouldn't surprise me if there's a similar phenomenon in the visual domain, where some set of parameters can be used to make a "faux-autochrome" mask that looks neither excessively random nor excessively orderly.