Long tail keywords refer to keyword phrases that offer a greater amount of information about what is being searched. They are designed to attract people typing longer, more descriptive sentences in the search query box. The theory is that people using long tail keyword phrases to search, as opposed to the shorter one to three word phrases, are much further along in their buying cycle and are therefore more likely to convert (take the desired action such as buy the product) because they have found a match for the exact thing they are searching for.
Long tail phrases can be particularly beneficial to a well-managed search engine marketing campaign, where on-going keyword analysis uncovers long tail keyword phrases already being used to search for what the advertiser offers. As a part of the campaign, the long tail phrases can be used to strategically compose highly relevant text ads that exactly match the query made by the end-user. Additionally, these types of phrases typically require lower bids to rank high in the sponsored results and therefore the CPC (cost per click) charges may be lower as well.