Search for anything using your favourite search engine. Almost instantly, the search engine will sort through the millions of pages it knows about and present you with ones that match your topic. The matches will even be ranked so that the most relevant ones come first.
Of course, the search engines don't always get it right. Non-relevant pages make it through, and sometimes it may take a little more digging to find what you are looking for. But, by and large, search engines do an amazing job.
As WebCrawler founder Brian Pinkerton puts it, "Imagine walking up to a librarian and saying, 'travel.' They're going to look at you with a blank face."
Unlike a librarian, search engines don't have the ability to ask additional questions to focus the search. They also can't rely on judgement and past experience to rank web pages, in the way humans can. Intelligent agents are moving in this direction, but there's a long way to go.
So how do search engines go about determining relevancy? They follow a set of rules, with the main rules involving the location and frequency of keywords on a web page. Call it the location/frequency method, for short.
Location, Location, Location...and Frequency
Remember the librarian mentioned above? He needs to find books to match your request of "travel," so it makes sense that they first look at books with "travel" in the title. Search engines operate the same way. Pages with keywords appearing in the title are assumed to be more relevant than others to the topic.
Search engines will also check to see if the keywords appear near the top of a web page, such as in the headline or in the first few paragraphs of text. They assume that any page relevant to the topic will mention those words right from the beginning. Frequency is the other major factor in how search engines determine relevancy. A search engine will analyze how often keywords appear in relation to other words in a web page. Those with a higher frequency are often deemed more relevant than other web pages.
Spice In The Recipe
Now it's time to qualify the location/frequency method described above. All the major search engines follow it to some degree, in the same way cooks may follow a standard chilli recipe. But cooks like to add their own secret ingredients. In the same way, search engines add spice to the location/frequency method. None do it exactly the same way, which is one reason why the same search on different search engines produces different results.
To begin with, some search engines index more web pages than others. Some search engines also index web pages more often than others. The result is that no two search engines has the exact same collection of web pages to search through.
Search engines may also give web pages a "boost" for certain reasons. For example, Excite uses link popularity as part of its ranking method. It can tell which of the pages in its index have a lot of links pointing at them. These pages are given a slight boost during ranking, since a page with many links to it is probably well-regarded on the Internet. Some hybrid search engines, those with associated directories, may give a relevancy boost to sites they've reviewed. The logic is that if the site was good enough to earn a review, chances are it's more relevant than an un-reviewed site.
Meta tags are what many web designers mistakenly assume are the "secret" to propelling their web pages to the top of the rankings. HotBot and Infoseek do give a slight boost to pages with keywords in their meta tags. But Lycos doesn't read them at all, and there are plenty of examples where pages without meta tags still get highly ranked. They can be part of the recipe, but they are not necessarily the secret ingredient.
Search engines may also penalise pages or exclude them from the index, if they detect search engine spamming. An example is when a word is repeated hundreds of times on a page, to increase the frequency and propel the page higher in the listings. Search engines watch for common spamming methods in a variety of ways, not least by following up on complaints.
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