How do search engines work?
Search engines help people find relevant information on the Internet. Major search engines maintain huge databases of web sites that users can search by typing in some text.
To compile their databases, search engines rely on computer programs called "robots" or, more specifically, "spiders." These programs "crawl" across the web by following links from site to site and indexing each site they visit. Each search engine uses its own set of criteria to decide what to include in its database. For example, some search engines index each page in a web site, while others index only the main page.
Also unique are the criteria each individual search engine uses to organize information for its users. Some list the results of a user's search according to which sites have the most links from other sites -- a system known as link popularity. Other search engines prioritize results according to the summary information contained in sites' meta tags, and still others look for common themes used throughout a site. There are many other ways to organize results, and most search engines use a combination of several of them.
Different engines, different rankings
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. If the first one weights meta tags as 70% of a site's value and keyword frequency as 30%, and the second one does the opposite, their results will be completely different.
Understanding ranking factors
Before you can understand how each search engine scores web sites, you need to understand the various factors they consider. These factors can be divided into two categories: page-related and outside.
