An important marketing concept known as "The Long Tail" is used to describe the hundreds to thousands of keywords and key phrases that a website is found for, yet rarely noticed or exploited by owners of the website.
The principle of the Long Tail is the opposite of focusing on the top 10-20 keywords for marketing your website. The "top keyword" concept is reinforced by agencies that contract to gain rankings for 10-20 terms, maybe 30. However, when studying the referrals from the search engines and the traffic they generate, those that focus on the top 10-20 terms may be missing the majority of their market.
People tend to focus on the thousands of visitors that come to the site for the most popular terms. Most site managers are very happy to see the numbers increase for those specific terms, and even happier to see those terms consistently ranking well. Conventional thinking applies the 80-20 rule that the top terms provide 80% of the business, but in evaluating multiple sites, this has proved to be the opposite.
In most cases, the top 10 terms provide a lot of traffic, but not nearly as much as the total terms after the top 10 or 20 most popular. Add up the terms that refer 1-3 visits during the month, and chances are, they will add up to more total visitors than the top terms. On closer examination, most sites will have the majority of their business (sales and leads) generated from these terms that are rarely tracked. This is the heart of the long tail -- the length, or total number of low-number referred terms outnumbers the height, or the total of top 10 terms.