SEO keywords, also known as keyphrases, are terms added to online content to improve search engine rankings for those terms. There are different types of SEO keywords that can be categorized by length, role, target audience, and other attributes. Some of the main types discussed are short-tail, long-tail, and mid-tail keywords defined by their length in words, as well as focus keywords, secondary keywords, and semantic keywords defined by their intended role and relationship to the main topic.