The longer the search phrase the more focused the searcher. You don't find a 56 inch Panasonic HD LCD television by searching for "TV's"
Longtail keywords contain 3 and 4-keyword phrases. Usually used by customer trying to be more specific to what they are really looking for. Also used to rank faster. Example, instead of tv show, use Charmed TV Show.
Thats a technique used for getting serp traffic fast since getting traffic from more competitive small keywords are harder.
Check my sig. I show you exactly how to find these low-competition/high search-count keyword phrases.