SEO is a technique that helps search engines find and rank websites higher in search results for relevant queries. It involves optimizing websites to be crawler-friendly and identifying keywords that best describe the website's content. Search engines work by crawling websites to index their content, processing search queries to calculate relevance scores, and retrieving results. While the basic process is the same, different search engines weigh various factors like keyword density and links differently, so optimization needs to be tailored for each engine.