There is a fundamental difference in the popular databases listing Internet pages between search engines and directories.
Search engine data is captured automatically while directories are collected and catalogued by editors.
Search engines use so-called search-robots (Robots, Crawler or Spider) to scour the Internet and fill their own databases.
These programmes read all of the relevant data on an Internet page and change to other Internet pages via links in order to virtually fathom the whole of the Internet automatically and save all relevant data in its own index.
Not only the size of the indexes varies from search engine to search engine but also the way in which the pages are indexed in each search engine database.
Every search engine uses its own algorithm to find and list Internet pages. This means that each search engine considers and assesses different factors in web-sites.
Therefore, an extensive knowledge of all search engine algorithms and their indexing procedures is essential to achieve the high positioning of your own web-site. However, search engines change their algorithms regularly and so a permanently high position can only be maintained by adjusting your own page accordingly.
When you enter a keyword in a search engine, the software analyses the search engine index using its own specific algorithm and then lists the results in the order of relevance.
Furthermore, search engines successfully use search directory entries as a quality indicator for ranking. In other words: being in the most important search index speeds up admission to a lot of search engines.