Search engine optimization (SEO) involves making small modifications to a website to affect its visibility in organic search results. Key on-page SEO elements include optimizing page titles, meta descriptions, URLs, content, images, and use of heading tags. Off-page factors involve improving site structure, link building, and submitting sitemaps and XML files. Google's Webmaster Tools help webmasters control how Google interacts with their site and provides data on crawl issues and search traffic.