Every search engine uses its own unique formula, called an algorithm, to index and score web sites. Search engines' algorithms weigh various factors, such as a page's design and links, to rank pages in their search results. By constantly refining and improving their algorithms, search engines hope to give their visitors the most relevant results.
Many search engines form partnerships and buy technologies to improve their algorithms.
They combine many factors and place different weight on each one. Webmasters who want their sites to achieve a Top 20 position spend a lot of time studying different algorithms. Even when several search engines rely on similar factors in composing their rankings, they may still rank the same site differently. For example, imagine two search engines both use meta tags and keyword frequency as their only ranking factors. Before you can understand how each search engine scores web sites, you need to understand the various factors they consider.