Long-tail keywords are:

a. Multi-word phrases that are extended or more specific versions of core fat head terms.
b. Terms with high relevance, high demand and low competition.
c. Fat head terms with high search volume Generic search terms.

a. Multi-word phrases that are extended or more specific versions of core fat head terms.

Explanation:

In website Search Engine Optimization (SEO) keywords has the main role to rank the website on higher positions. The keywords should be relevant to the content that website provide to its users.

Long term keyword are the keywords, that contains multiple phrases or words to make a single keyword. These keywords are more specific to catch the user need.

For Example:

If some one have online mobile shop, he want to make a website to facilitate his customers whenever the are searching for the mobile. If he use only short keyword that contains one or two words, the competition of the search is high.  That keywords are almost used by most of the websites.

If key word is "Samsung Mobile", it is a short tail keyword, that may not rank the new website. because this is not specific.

"Low price Samsung Mobile S5 available online" this keyword is a long tail keyword contains more information of your website buisness. If someone want S5 in low price and want to purchase online he can access your website easily.

a  social news website  is an  internet  website  that features  user-posted stories. such stories are ranked based on popularity, as voted on by other users of the site or by website administrators. users typically comment online on the news posts and these comments may also be ranked in popularity. since their emergence with the birth of  web 2.0, social news sites have been used to link many types of information, including news, humor, support, and discussion. all such websites allow the users to submit content and each site differs in how the content is moderated. on the  slashdot  and  fark  websites,  administrators  decide which articles are selected for the front page. on  reddit  and  digg, the articles that get the most votes from the community of users will make it to the front page. many social news websites also feature an online comment system, where users discuss the issues raised in an article. some of these sites have also applied their voting system to the comments, so that the most popular comments are displayed first. some social news websites also have a  social networking  function, in that users can set up a user profile and follow other users' online activity on the website.

a  social news website  is an  internet  website  that features  user-posted stories. such stories are ranked based on popularity, as voted on by other users of the site or by website administrators. users typically comment online on the news posts and these comments may also be ranked in popularity. since their emergence with the birth of  web 2.0, social news sites have been used to link many types of information, including news, humor, support, and discussion. all such websites allow the users to submit content and each site differs in how the content is moderated. on the  slashdot  and  fark  websites,  administrators  decide which articles are selected for the front page. on  reddit  and  digg, the articles that get the most votes from the community of users will make it to the front page. many social news websites also feature an online comment system, where users discuss the issues raised in an article. some of these sites have also applied their voting system to the comments, so that the most popular comments are displayed first. some social news websites also have a  social networking  function, in that users can set up a user profile and follow other users' online activity on the website.