News Courtesy of Yoast.com:
You’ve probably heard that doing keyword research is an essential, first step in optimizing your website properly. But how about these keywords: can they only be single words or can they also be longer multi-word keyphrases? And what exactly is the difference between the two? In this SEO basics post, we’ll explain the difference between a keyword and a keyphrase.
What are the differences?
Keywords or keyphrases should both describe the essence of what the post is about. The difference between the two is that keywords are single words, while keyphrases are made up of a few words.
Ryan’s Take 
Most veteran SEO’s should already know the difference. However, there is something to be said about long-tail keywords. Wouldn’t they just be the same as a keyphrase? The idea for both is that multiple words are used to target a specific query. According to Yoast, they are not exactly the same.
One analogy that can be used is that of a square and rectangle. A square is a rectangle but the opposite can not be said. The same philosophy holds true in regards to long-tail keywords and keyphrases. Keyphrases can either be specific or general. Contrast that to long-tail keywords which are always specific.
An example of each would be as follows: Keyphrase (General) Dog Food. Keyphrase (Specific) Dog Food for Chihuahua’s. Long-tail Keyword Dog Food for Smooth-Coat Chihuahua. As you can see, the long-tail keyword is super detailed and proper optimization will yield great SEO results. The question is whether that effort is better spent on targeting many long-tail keywords or one keyphrase. I suppose that would depend on the industry that is being targeted and the amount of competition there is.
