To some, the concept of the Long Tail may already be cliché. To others, it may already be passé. To most people, however, the concept is still alien—only a first-time read, perhaps on sites like this one.
The Long Tail principle is quite a new term in the lexicons of businessmen. It borrows from the statistical principle of the same name, which basically states that the distribution of lower-frequency or lower-magnitude items can usually outweigh that of higher-frequency or higher-amplitude items. Translating to simple business terms, you will earn more from selling the more obscure or cheaper items at volume than selling a few bestsellers at high prices.
For retail companies, this means popular items are good. But one can earn better by selling lots of obscure, niche-oriented items, which more people tend to seek out and purchase.