Many things ranking tools
Search engines use programs called spiders or crawlers to gather the information on web pages. The information they look through is not just the information in front of the eyes of the viewers, it is also the underlying code of the web page. When building a web site it is best to have an expert Seo service help you incorporate the concept into your site. For web sites that have already been built they can be optimized. When optimizing an existing site, the site usually has to get modified.