A Keyword refers to a word, phrase or sentence used in online search. (in a nutshell, whatever you would type into google’s search query box)
A Keyword can be a short-tail keyword (can be as short as a word) or a long-tail keyword (could be a phrase or a sentence). Short-tail keywords are broader (i.e. television). Long-tail keywords are more specific, and make for much more targeted searches and results (i.e. sony 55″ lcd bravia television best price). 70% of all searches performed are Long-tail keywords. Long-tail keywords also catch people when they are closer to purchasing something.