What’s a Long Tail Keyword?
Essentially, long-tail keywords are less popular keywords because they have less search volume and less competition to rank for. Consider the following two examples: “home remedies for bed bugs” or “how to get rid of depression.” These are each considered long-tail keywords as compared to trying to rank for the much more competitive search terms “bed bugs” or “depression”.
From an authoritative article at SearchEngineWatch
Long-tail keywords are longer and more specific keyword phrases that visitors are more likely to use when they’re closer to a point-of-purchase. They’re a little bit counter-intuitive, at first, but they can be hugely valuable if you know how to use them.
Take this example: if you’re a company that sells classic furniture, the chances are that your pages are never going to appear near the top of an organic search for “furniture” because there’s too much competition (this is particularly true if you’re a smaller company or a startup). But if you specialize in, say, contemporary art-deco furniture, then keywords like “contemporary Art Deco-influenced semi-circle lounge” are going to reliably find those consumers looking for exactly that product.
Another intelligent article at wordstream.com