We often think that if we can rank for the largest number of searchers for a core search term that we are winning the SEO wars. Fact is the conversion rate can be as much as 2.5x more for a "long tail" term as apposed to the core "head term" search word.
Conductor, who offers a great service called Searchlight, has put together a simple two page review of their findings on this and can be had for free at:
http://www.conductor.com/resource-center/research/long-tail-search
