Understanding the phrases people use in searching for content can make the difference in being found, or not.
Late 2013 Google killed the free Google Adwords Keyword Tool, negating all the advice saying to start there. That tool was to be used by people planning adwords campaigns, so they'd know what keywords to target. But I suppose it was abused by SEO's looking for a cheap way to access search phrase data. In anycase that's not the point of this page - the purpose of this page is to list out tools to help you learn about search phrases being used by real people.
The theory is that content closely matching typically used search phrases - matching the search requests people type into search engines - the more likely your content will show high in the search results. The higher your page ranks, the more traffic you're likely to get.
The Google Webmasters Tools tells a growingly rich amount of data about how Google understands your website. One of the reports is the keywords for which your website ranks, the average rank, the search traffic for each keyword, and the amount of that traffic you're snagging. This is great data to guide parts of your site which need improvement - such as, if your page is snagging 0% of traffic but ranks #2 in results, there's something wrong with the page. It doesn't tell you what, but isn't it useful to know which pages are in most need of changing?
This tool doesn't let you browse the other search phrases being used.
Free keyword tools Wordtracker - https://freekeywords.wordtracker.com Ubersuggest � http://ubersuggest.org/ SEO Book � http://tools.seobook.com/keyword-tools/seobook/ WordPot � http://www.wordpot.com My favorite paid options MOZ � http://moz.com/tools/keyword-difficulty Raven Tools � http://raventools.com/tools/research-central/ KeywordSpy � http://www.keywordspy.com/ SEMRush � http://www.semrush.com/