SEO is the process of modifying a website to increase its ranking in organic search results. Most search engines use spiders that crawl the web and index pages based on keyword occurrence and link popularity. There are three main types of search engines: crawler-based like Google, human-powered like Yahoo, and hybrids like MSN Search. Spiders look at HTML code and structure rather than images or design. Effective SEO techniques include optimizing on-page factors like titles, headers, descriptions and keywords as well as off-page factors like links and social signals while avoiding black hat techniques.