Search engines help people find relevant information on the Internet. Major search engines have huge databases of web sites that surfers can search by typing in some text.
Search engines send out spiders or robots, which follow links from web sites and index all pages they come across.
Each search engine has its own formula for indexing pages; some index the whole site, while others index only the main page. Search engines decide the amount of weight that will be placed on various factors that influence results . Some want link popularity to be the most important criterion, while others prefer meta tags.
Search engines use a combination of factors to devise their formulas