Search engines are internet tools that search an index of documents for user-specified terms. They work by having "spiders" crawl the web to find new documents and index their content and metadata into databases. When a user searches, the search engine searches its own database to find and rank relevant documents using algorithms that consider factors like keywords, links, and page popularity. Most search traffic and sales are driven by search engines, so businesses use SEO techniques like optimizing content, code, and off-site links to improve their search visibility and rankings. Proper SEO requires an understanding of how to make content both human-readable and search-engine friendly.