Difference between overdrive, distortion, and fuzz:
Overdrive, distortion and fuzz are often used interchangeably, but in a nutshell can be used to describe three degrees of the same effect: overdrive being a slightly grittier tone often heard in tube amps, distortion being a much dirtier tone while still retaining note definition, and fuzz tends to be a much more extreme distortion where note definition begins to be lost.
On a technical basis: overdrive refers to soft clipping, whereas the extremes of a sine wave are compressed a bit, distortion compresses them significantly more, and fuzz cuts off the tips. In the diagram below, overdrive and distortion are more characterized by the yellow, the red shows fuzz and the blue denotes the original signal.
http://users.chariot.net.au/~gmarts/pix/f_clip.gif