The frequency synthesis approach to terrain synthesis is based
on the observation that fractional Brownian motion (fBm) generates curves
and surfaces that look a lot like coastlines and mountains.
Because fBm noise has a well defined power spectrum, random frequency
components can be calculated fairly easily, and the inverse Fast Fourier
Transform then converts the frequency components into altitudes.
An example applet using this technique can be found here.

Because frequency synthesis uses the Fourier transform, terrains
generated by this method will wrap around both horizontally and vertically.
This is fine if you want to seamlessly tile a surface, but it can be
a problem if you are trying to generate a small area. You can avoid
this wraparound problem by generating a larger heightfield and discarding
areas near the edge, but this requires extra work.