“Long tail” refers to the marketing concept that describes the strategy of selling a large number of unique items in quantity ‘relatively low – usually with a parallel activity of selling few items in large quantities.
The concept was popularized by an article by Chris Anderson in Wired (October 2004), in which he mentioned Amazon.com and Netflix as examples of business built around this principle.
Anderson’s thoughts on the issue then becomes a blog and finally a book: “The Long Tail: Why the Future of Business Is Selling Less of More“, Hyperion, 2006
If you are interested observer to the world of design from this perspective (starting from the economic data), we suggest you read on a constant base The Economist and the Business Week (especially its “innovation” section”). Their analysis and reflections are generally the best ones you can find around.