Search terms are the words and phrases that people type into the search forms of search engines. Keywords and keyphrases are the words on your site that match these search terms.
Before you do any type of search engine marketing, you have to understand your target audience and know the search terms they are using. Once you know the search terms being used, these can be included in the content of your Web site as keywords and keyphrases.
Your keywords and keyphrases will also be used if you plan to promote your site with pay per click or keyword based pay per impression advertising.
To learn more about Keywords and Search Terms: