Crop Factor

The crop factor [1] comes from the fact that the Sensor in a digital camera is smaller than a frame of 35mm film. This means the digital slr crops out the center portion of a given lenses field of view, but the image is still at the full resolution of the digital sensor. This crop factor gives a lens on a digital SLR a smaller field of view than it would have on a film SLR, giving it the same field of view of a longer lens when compared to a film SLR.
[1]http://www.dcresource.com/forums/archive/index.php/t-3937.html
[2]http://www.millhouse.nl/digitalcropfactor.html
[3]http://www.luminous-landscape.com/tutorials/understanding-series/dslr-mag.shtml