Maxim offers a great explanation of DC-DC converters.
DC-DC Converter Tutorial
Abstract: Switching
power supplies offer higher efficiency than traditional linear power
supplies. They can step-up, step-down, and invert. Some designs can
isolate output voltage from the input. This article outlines the
different types of switching regulators used in DC-DC conversion. It
also reviews and compares the various control techniques for these
converters.
http://www.maxim-ic.com/app-notes/index.mvp/id/2031