How Search Engines Work

When a searcher enters a query, the search engine starts by analyzing the query, examining the words for misspellings or word variants that should also match the exact words entered.

The search engine also eliminates or downgrades the important of commonly-used words (known as stop words), such as "of" and "the." During this step, the search engine processes search operators (such as "+" or "-") and notes the word order of the query.

Basically, the query analysis determined what words to look up-not just the words that were typed as the query, but any word variants (mouse and mice)-and which to ignore (stop words and antiphrases).

The engine goes to work looking up each word in the query to see which pages contain the word. (The search index can be thought of as an alphabetical list of every word that occurs on every page of the Web.) So, when you look up the word "glaucoma," you get a list of every page that contains that word.

That's the simplest case. It's more complicated when searchers enter more complex queries. If the searcher was looking for "glaucoma treatment," then the engine would look up every page that contains each word, giving it a list of pages that contain "glaucoma" and a list of pages that contain "treatment."

Most search engines, faced with this decision, decide to show just the pages that contain both words. So they look through the two lists and find which Web pages are listed in both.

Merely displaying the list of all the pages that contain the words in the query isn't much help when there are more than a dozen pages.

And with Web search, that's almost always the case. So, one of the most important parts of a search engine is the ranking algorithm-the part of the search engine that decides which pages show up at the top of the results list.

Ranking is just another word for sorting, the act of collating results into a certain order.

For some kinds of information, such as news stories, ranking results by the date of the in-formation (newest first) may make sense, but most organic search results are ranked by relevance, the degree to which the pages match the subject of the query.

Ranking algorithms usually take into account keyword density (the number of keyword occurrences compared to the total number of words on a page), keyword prominence (how close to the top of the page the kwywords occur), link popularity (how many other Web pages link to a page and how important they are), and many more.

Organic search results look very similar no matter what search engine you use. They all use the title of the page followed by a snippet--a summary of the text from that page that contains the search terms.

The search terms are usually displayed in bold, drawing the searcher's eye to them. You should understand that everything displayed in the search results is drawn from what the search engine previously stored in its index.

The search engine never examines the actual page while it is displaying search results, which is why the results page can sometimes contain outdated information, or even display pages that no longer exist (which are discovered when searchers click them).

The information displayed on the results page was correct when the spider last crawled the page, but the page may have changed (or even been removed) since.