Chris Anderson - The long tail theory, SELLING LESS OF MORE:

Chris Anderson - The long tail theory, selling less of more:
"The Long Tail or long tail refers to the statistical property that a larger share of population rests within the tail of a probability distribution than observed under a 'normal' or Gaussian distribution. The term has gained popularity in recent times as a retailing concept describing the niche strategy of selling a large number of unique items in relatively small quantities – usually in addition to selling fewer popular items in large quantities. The Long Tail was popularized by Chris Anderson in an October 2004 Wired magazine article, in which he mentioned Amazon.com and Netflix as examples of businesses applying this strategy. Anderson elaborated the concept in his book The Long Tail: Why the Future of Business Is Selling Less of More 
The distribution and inventory costs of businesses successfully applying this strategy 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 total sales of this large number of "non-hit items" is called the Long Tail."
e.g. iTunes, Amazon, Play.com

In his own words:
"The Long Tail, in a nutshell
The theory of the Long Tail is that our culture and economy is increasingly shifting away from a focus on a relatively small number of "hits" (mainstream products and markets) at the head of the demand curve and toward a huge number of niches in the tail. As the costs of production and distribution fall, especially online, there is now less need to lump products and consumers into one-size-fits-all containers. In an era without the constraints of physical shelf space and other bottlenecks of distribution, narrowly-targeted goods and services can be as economically attractive as mainstream fare.
One example of this is the theory's prediction that demand for products not available in traditional bricks and mortar stores is potentially as big as for those that are. But the same is true for video not available on broadcast TV on any given day, and songs not played on radio. In other words, the potential aggregate size of the many small markets in goods that don't individually sell well enough for traditional retail and broadcast distribution may someday rival that of the existing large market in goods that do cross that economic bar.
The term refers specifically to the orange part of the sales chart above, which shows a standard demand curve that could apply to any industry, from entertainment to hard goods. The vertical axis is sales; the horizontal is products. The red part of the curve is the hits, which have dominated our markets and culture for most of the last century. The orange part is the non-hits, or niches, which is where the new growth is coming from now and in the future.
Traditional retail economics dictate that stores only stock the likely hits, because shelf space is expensive. But online retailers (from Amazon to iTunes) can stock virtually everything, and the number of available niche products outnumber the hits by several orders of magnitude. Those millions of niches are the Long Tail, which had been largely neglected until recently in favor of the Short Head of hits.
When consumers are offered infinite choice, the true shape of demand is revealed. And it turns out to be less hit-centric than we thought. People gravitate towards niches because they satisfy narrow interests better, and in one aspect of our life or another we all have some narrow interest (whether we think of it that way or not)."


Basically, the long tail theory demonstrates that the internet is able to support online stores that sell niche products, those that are not widely available as they are not in great demand. But by selling a small number of a lot of products, they are able to sell as much as physical stores. Stores are not able to stock such niche products, they have to go with what will sell the largest amounts, this is why shops such as HMV sell mainstream products. However the rise in use of iTunes has created huge competition for HMV, less people purchase physical cd's and albums favoring downloads. By offering more niche products consumers are able to gravitate towards their own niche, which makes them more likely to continue buying products of that niche and less likely to go back to mainstream.
- physical stores have to go with what will produce the highest number of sales as they have limited shelf space, whereas online stores like amazon can stock millions of different products as they can store it all in warehouses etc. 
This has created a shift from mainstream to niche, which would have been a lot less likely as niche products were so hard to come across before the revolution of such online stores.

The Long Tail book is about the big-picture consequence of this: how our economy and culture is shifting from mass markets to million of niches. It chronicles the effect of the technologies that have made it easier for consumers to find and buy niche products, thanks to the "infinite shelf-space effect"--the new distribution mechanisms, from digital downloading to peer-to-peer markets, that break through the bottlenecks of broadcast and traditional bricks and mortar retail.