It's hard to find a short definition of "The Long Tail." You can read the original article in Wired, but here's a good summary by Joe Gratz, a law student studying digital rights:
In it [The Long Tail article], Anderson argues that new dissemination technologies make relatively unpopular works a much more important source of revenue. For example, even the largest chain bookstores can stock only a fraction of the titles available on Amazon.com; even the largest music stores can stock only a fraction of the tracks available on Rhapsody. If you make a graph with the number of sales on the vertical axis and arrange items by their sales rank along the horizontal axis, you get a graph with a large peak at one end, representing the popular, mainstream items that sell many copies each. But from there, the graph drops steeply, then trails off very slowly – a “long tail” of items that each sell very few copies, but that in aggregate could represent a substantial proportion of overall sales.