Longtail keywords in a few words: Longtail keywords are long and specific keyword phrases that generate little traffic and few conversions each. Long tail keywords usually consist of several words and they have a low monthly search volume (in most cases).
Longtail keywords study:
The Rimm-Kaufman Group published a study about longtail keywords. Google AdWords advertisers with monthly expenses ranking from $15K to $750K per month were analyzed in the study. All of these advertisers had well managed Google AdWords accounts.
Results of the study:
1. The importance of longtail keywords varies. For one company, the long tail keywords generated only 8% of the sales. Another company received 83% of its sales through longtail keywords.
2. It depends on the product. People search differently for different types of products. For one product group people might use more longtail keywords while they use fewer for another product group.
3. The importance of longtail keywords does not depend on the paid search budget.
4. Longtail keywords are most important for SKU-based commodity retailers.
5. For some Google AdWords advertisers, longtail keywords are critically important. One company in the test generated more than 71% of its revenue from longtail keywords.
Longtail keywords are important to all AdWords advertisers. Even the company that received only 8% of its sales through the longtail keywords made a significant amount of money with them. Go after the longtail keywords people 
Long tail keywords are more and more used in Internet Marketing and e-commerce as they represent a secure way of generating traffic. It is easier to rank for them and they have lower competition.