Long-tail keywords refer to the multitude of keywords which taken individually represent little search traffic but a big part of a website search traffic when considered all together. Long-tails can count hundreds or even thousands of keywords for a single generic short-tail keyword.
Long-tail keywords are usually longer than short-tail keywords and can be made of three or more words. But some rare synonyms can be one word long and being long tail keywords.
An illustration of long-tail keywords by Search Engine Partner:
Long-tail keywords concept is equally used for SEO and PPC search engine campaigns.
The use of long-tail keywords has two main benefits, the competion is less intensive - and bids are lower for PPC campaigns - and some long-tail keywords are used by highly qualified visitors.
Image source Mindsaw blog
For SEO context, the use of long-tail keywords requires often to multiply doorway pages by the way of automated production.
For AdWords campaigns, if not automated, long-tails keywords campaigns are time-consuming and the doorway pages possibly created for SEO purposes may be very useful.
Many tools and dedicated functions are offered for long-tail keywords searching and management.
See also short tail keywords.
An example of keyword research tools: