Long tail is a term invented by Chris Anderson in 2004 to adapt the famous Pareto law to search engine optimization of web pages.
Applying Pareto's law in terms of market, we would conclude that many companies have few star products that get a lot of sales while other products, say side, that target market niches have poor sales individually, so with 20% of the products get 80% of sales. Why then strive to offer products minority if we can attack the big cake?. If a bookstore with 20% of the books for 80% of its sales, how to provide highly technical books for a small niche market?
But the Internet has facilitated overcome this barrier. Amazon, the most famous online store, is the classic example of using the long tail, the Amazon website is truly competitive because it can have a catalog of books far superior to its competitors off-line. So the company makes more money with fewer sales of many different specialized books or minority with the usual bestsellers.
In addition, the advance of technology has made for many companies is customary that the total sales of secondary products far exceed that of the star products.
Another example? Google itself. If we analyze such Adwords ads for eg "web design", there are so many advertisers, in fact many more than for "web design Barcelona", "web design Madrid", "design online stores" ... but where it really gets Google benefits is the sum of these "small" words that far exceeds the word star "web design".
In terms of positioning or seo, the long tail has a very clear. Better to concentrate on positioning a website for multiple keywords for which we can get the top positions in the browser that can not concentrate all our resources on a few key words very contested. Eventually we'll get more visitors and better results. And besides, there is evidence that these words get a higher conversion rate.
This strategy is especially useful for e-commerce or online stores.