Mark Crofton said..
The Long Tail pertains to a relatively simple concept, that capturing all of the the small volume will result in a collectively large volume. With regards to the keyword long tail, the idea is that there are only a few words or phrases that receive a large number of searchers, but there are a huge number of obscure phrases or keywords that receive a small number of searches each month. By capturing top ranking for all of the less popular terms you could in effect receive more traffic than you would if you had top ranking for the popular terms.
Capturing the keyword long tail, is very difficult. It requires massive amounts of unique content. Webmasters have generally found the easiest way to generate unique copy is through user-generated content (ie. forums, blog comments, or user reviews).