Keywords are popular search terms that can have short or long tails for a given topic. A short tail is one or two words that will return a broad range of information. They are very popular and because of that website content is generally created with these words in mind.  A Long tail phrase contains a few more words which narrows the focus. They are less popular but still widely used. Here's a graph that illustrates this point:

On the left, many people are searching a topic with one or two keywords. On the right, there are fewer people using many different long tail phrases.  Notice the area of the long tail. It's a considerable percentage of the total.  A site that is optimize for just one short tail keyword phrase is not taking full advantage of overall demand.

To optimize for the Long Tail, develop web pages with one long tail keyword phrase in mind for each page, used in the main text, image descriptions, links, page title, paragraph headings, and domain name.  Search engines will index these pages based on the phrase and attempt to list them when enough of the words are used in a search query.

Here is the Goggle Keyword Research Tool. Type in a phrase that applies to your interest and see how popular it is compared to similar ones of varying lengths.