The keywords you want to focus your SEO on should closely resemble the vocabulary of your audience. In order to come up with the proper keywords you really have to get inside the heads of the people who search for your website. What terms will people use? How do people search? Which question does your website answer? You should create a list of all search terms people could use and think of combinations and nuances within these search terms.
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Long Tail Pro is a keyword research tool that helps you quickly find low competition keywords for your website. In particular, the tool is built for people doing SEO (search engine optimization) as opposed to people doing paid ads (Google Adwords PPC). I'm sure there are some advertisers that use the tool for paid ad keyword research, but the tool was built for SEO keyword research.