Clayton Christensen breaks technology developments into two groups. "Sustaining" technology improvements are those which seem to be in line with the needs of the current customers. The "Disruptive" technology developments are those which don't immediately seem to meet the established customer's demands. The basic problem is that often the large companies get blindsided by disruptive technology, and they either don't react, or react too late. Normally someone will figure out a slightly new market for the disruptive technology, improve the basic process, drive down the costs, and then go after the more established markets.
Clayton shows how this model applies to a number of different industries over the last hundred years. He has a lot of hard data from the Disk Drive industry, but he mentions a number of other industries which underwent fundamental changes, and shows how the old companies were not able to adapt to the changes.
"The Innovator's Dilemma" and "Crossing the Chasm" go well together. They are addressing many of the same issues from different perspectives. Clayton Christensen is looking at changes in the market place from the point of view of the large established companies, while Geoffrey A. Moore is coming from the other direction, that of a small startup company with some new cool technology. Clayton is more concerned with technology, while Geoffrey is focused on more on marketing issues.
This is a very well written book, good for anyone in business. It reads well, and I found it very thought provoking. There are a lot of good ideas here.