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.