The phrase The Long Tail was first coined by Chris Anderson in an October 2004 Wired magazine article to describe the niche strategy of certain business such as Amazon.com or Netflix. The distribution and inventory costs of those business allow them to realize significant profit out of selling small volumes of hard-to-find items to many customers, instead of only selling large volumes of a reduced number of popular items. The group of persons that buy the hard-to-find or "non-hit" items is the customer demographic called the Long Tail.
Source: Wikipedia http://en.wikipedia.org/wiki/Long_Tail

Additional recommended sources:

The Long Tail" by Chris Anderson, Wired, Oct. 2004 http://www.wired.com/wired/archive/12.10/tail.html

"Goodbye Pareto Principle, Hello Long Tail" by Erik Brynjolfsson, Yu (Jeffrey) Hu, and Duncan Simester, December 2006 http://ssrn.com/abstract=953587

"A practical model for analyzing long tails" by Kalevi Kilkki, First Monday, May 2007. http://firstmonday.org/issues/issue12_5/kilkki/index.html