What are long-tail keywords?

A "long tail" keyword is a phrase very specific to what your website is selling and what people are looking for. By comparison, shorter keywords are called "head keywords". Example:

Head keyword: laundry detergent.

Long-tail keywords: environmentally safe laundry detergent.

Long-tail keywords are usually cheaper to bid on for Google AdWords campaigns because there is less competition for them.

Also, people who search online using long-tail keywords are far more likely to become buyers because they know exactly what they want and are often searching for a supplier.

By comparison, people who search on the head keyword are typically more in the research stage and not so ready to buy.

Source: Wordtracker, 7/17/12