Websites like Google, Yahoo and Bing that function as a way of prioritizing the websites, and information they contain, for us. (Think library card stacks.)
The process of paying a search engine to appear on their webpage next to specific key words.
The hierarchy created by search engines ratings of websites and the websites relevance to a specific term, phrase or keyword.
The process of optimizing website content to give your website higher status in the hierarchy of Organic Search.
The process of marketing (or advertising) with the search engines for paid placement next to specific terms and phrases.