The "Long Tail" is a term thrown around a lot by Internet entrepreneurs and marketeer/philosophers like Doc Searls as a means to describe the rediscovered sales base across all industries.
During the early and mid-90's the notion of the blockbuster was everything. Films like Jurassic Park and Independence Day epitomised this concept of putting all your eggs in a couple of well defined markets and selling it to the public like there was no tomorrow. However the blockbuster ignores the common fact that for every exclusive, overpriced item (be it fashion label or computer) there is thousands of smaller, cheaper alternatives that don't sell as well individually but combined make up an overwhelming majority of industry sales.
The low cost, distributed nature of the Internet has allowed these smaller brands to sell their wares on an equal footing with more established name-brands. This exploding market has been termed the 'Long Tail' and is the target of 99% of Internet shopping sites and the subject of this very good short video.