Long tail keywords are basically a keyword phrase with more than 2 or 3 keywords.
For example the keyword ‘edible oil’ cannot be considered as a longtail keyword but ‘edible oil refining process India’ surely is.
A good SEO starter plan should surley target the longtail keywords first as getting the rankings for these kind of keywords is comparitively faster so the website starts getting the web presence which gradually improves with time.
I have been following the philosophy of long tail keywords since 2004 from the SEO perspective.
If the site starts getting ranked for the long tail keywords then generic ones are sure to follow sooner or later once the site gets perfectly indexed.
Moreover, the search patterns of the searchers have also become very specific hence the long tail keywords shall surely bring in more targeted traffic.
The topic is excellently explained here http://www.webpronews.com/expertarticles/2010/03/10/longtail-seo-for-ecommerce