An algorithm was named after a Iranian mathematician, Al-Khawarizmi and is a sequence of steps to achieve a certain goal. Algorithm's are primarily used by search engines to order web site search results. For example Google uses an algorithm to try and find the most relevant web pages for search queries and also tries to stop spam. Therefore their algorithm is always changing and is therefore a (generic) algorithm. An algorithm can also be used loosely for any sequence of actions, whether it terminates or not.

Some of the things search engines use to rank a webpage,

1. Information in the Meta tags such as the meta description and keyword tags. And title tag.
2. The amount of websites linking to a webpage.
3. How fast a webpage loads and the amount of time to crawl it.
4. The size of a webpage, the smaller the better.

Modern search engines use their own techniques which may differ to the above, and are not made known to the public.