Search engines work by using software known as spiders or robots to survey the Web and build databases. Web documents, when retrieved, are analyzed and then added to a search engine’s index.
When you log onto a search engine site and enter a query, typing in typewriter for example, your input is then checked against all the pages the search engine has in its index. The pages, or Websites, that best fit your search criteria will be displayed ranked with those with the best match listed first.
If you want to narrow the search, use keywords. What is a keyword, exactly? It can simply be any word on a Webpage. For example, I used the word “simply” in the previous sentence, making it one of the keywords for this particular paragraph. However, since the word “simply” has nothing to do with the subject of this paragraph (i.e., how search engines work), it is not a very useful keyword.
Useful keywords and key phrases for this page, if it were a Website, would be “search,” “search engines,” “search engine methods,” “how search engines work,” “ranking relevancy,” “search engine tutorials,” etc. Those keywords would actually tell a user something about the subject and content of this article.
Unless the author of the Web document specifies the keywords for her document it’s up to the search engine to determine them. Essentially, this means that search engines pull out and index words that appear to be significant.
Since search engines are software programs, not rational human beings, they work according to rules established by their creators for what words are usually important in a broad range of documents. The title of a page, for example, usually gives useful information about the subject of the page (if it doesn't, it should!).
Words that are mentioned towards the beginning of a document (think of the "topic sentence" in a high school essay, where you lay out the subject you intend to discuss) are given more weight by most search engines. The same goes for words that are repeated several times throughout the document.
Some search engines index every word on every page. Others index only part of the document.
Keyword searches have a tough time distinguishing between words that are spelled the same way, but mean something different (i.e. hard cider, a hard stone, a hard exam, and the hard drive on your computer). This often results in hits that are completely irrelevant to your query.
Some search engines also have trouble with so-called stemming—i.e., if you enter the word “big,” should they return a hit on the word, “bigger?” What about singular and plural words? What about verb tenses that differ from the word you entered by only an “s,” or an “ed”?
Search engines also cannot return hits on keywords that mean the same, but are not actually entered in your query. A query on heart disease would not return a document that used the word “cardiac” instead of “heart.”
This is just a hint at how a search engine works. Remember when searching try to be specific.