Search engines works are divided into three major steps crawling, indexing and processing.
In first step, search engines use a special robot called spiders to create a list of words found in the web pages. This process is called crawling. Spiders crawl around the web to collect data. They enter every links found on a webpage. Crawling normally starts from most used server or a page which has a high popularity and visits all links found on the page.
All these collected data gathered on a huge database. But saving on database is not indexing . There are few days required to index or update a saved page.
Then all these indexed pages are processed to give ranking for a particular search. This ranking is given upon by considering several parameters. These parameters include quality of content, keyword density, anchor text, titles and subtitles. Another important parameter is the quality of links coming to this site. If a link from highly trusted sites, google considers this site is appropriate for the top position. Google give ‘pagerank’ to all sites in a scale 0-10. They consider the higher pagerank site is most trusted. They also count the number of incoming links from other sites.
Search engines always try to cover all the sites but there are some limitations. So a site should be optimized for search engines. Thus the jobs of search engine spiders become easier.
This is the cause to do SEO works. Every site should be well optimized to get maximum exposure.