This section contains articles and writings on in-depth research into search engine algorithms of ranking. Most commonly, these data were retrieved in course of SEO experiments and Systems Analysis of search engines carried out by various SEO labs.

Every search engine has its own relevancy algorithm (rules for ranking), which is continuously developing as search engines are learning from their experience, but let’s start from the defining search engine algorithm. It’s a complex mathematical formula used by a search engine to rank the webpages that it finds by crawling the Web. The search engines' algorithms are very top secret; there are many people who work outside of the search engine industry who attempt to reverse engineer the code so they can understand how they work in an attempt to help people to rank higher in the results. An algorithm is a set of rules that a search engine uses to rank the listings contained within its index in response to a particular query. No search engine reveals exactly how its own algorithm works to protect itself from competitors and those who wish to spam the search engine.

As search engines strive to improve the quality of their search results, search engine algorithms become more and more sophisticated. The historical development of Internet search engines ranking starts with the logic of an ordinary Web surfer and keyword distribution, but due to the development other factors such as “link popularity” prevailed and have remained huge factors in ranking.

Describing search engine algorithms will not be complete without differentiating for types of engines, which, of course, have distinguishing principles of ranking. The first type is crawler-based search engines such as Google and MSN – top search engines. They are also called “traditional.” Another type is directories that use human resource for indexing. Most directories do not have their own ranking mechanism; they use some obvious factor to sort the URLs such as alphabetic sequence or Google Page Rank.

The next type is hybrid engines, which include META engines and those using other engines’ results. The last type is PPC and paid inclusion engines that provide paid listings.

Crawler-based search engines apply a sophisticated technique to determine how relevant your pages are to search words and phrases. Though each search engine has its own distinguishing features, the main idea lies in regarding many on-the-page and off-the-page factors, and only after this is determined is your page given a certain position or rank.

Another situation is with PPC engines. Paid inclusion engines require certain fees to list your page as well as some difference for working systems such as re-spidering or top-ranking for keywords you choose. The fundamental principle that lies at the heart of PPC process is the that higher you bid, the higher your position will be for the particular search terms. In this system, keywords and phrases are associated with a cost-per-click (CPC) fee.

This well-made auction system is popular, and most major Internet search engines utilize such schemes as a part of their indexing and ranking system. Yahoo!, for example, owns Overture's pay engine and provides search results to AltaVista, AllTheWeb, MSN, Overture's supplemental results, and other Inktomi-powered engines such as HotBot. Sponsored results from Overture are found at the top, side, and bottom of the search results pages fed by Yahoo!, but, of course, this search engine ranks pages looking on their on-the-page and off-the-page factors such as MSN and Google top search engines.

Google search engine is one of the most widely know for it demands towards submitting pages. Though many consider Page Rank system Google’s main search engine algorithm, it has other ways to rank pages, too.

Page Rank is an absolute value that is regularly calculated by Google for each page it has in its index. Here, you should to know that the number of links you've got from other sites outside your domain matter greatly as well as the link quality.

There are two other terms related to Google's way to rank pages: Hilltop and Sandbox.

Hilltop is an algorithm that was created in 1999. Basically, it looks at the relationship between the "Expert" and "Authority" pages. An "Expert" is a page that links to lots of other relevant documents. An "Authority" is a page that has links pointing to it from the "Expert" pages.

In theory, Google would find "Expert" pages and then the pages that they link to would rank well. Pages on sites like Yahoo, DMOZ, college sites, and library sites can be considered experts.

Google also checks age of your page by help of Sandbox algorithm, which detects how old your page is and how long ago it was updated. Usually, pages with stale content tend to gradually slip down the result list while the new pages just crawled initially have higher positions than they would if based on Page Rank only. In other words, Google considers new pages to have more relevant and up-to-date content and gives them a certain advantage over the stale pages. Therfore, constantly updating your pages can help keep them up the list.

In conclusion, you should remember that search engine algorithms are complex and sophisticated mechanisms that are dangerous to play with if you have no proper notion of what search engines take into consideration while ranking your pages.