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HOW SEARCH ENGINES WORK
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How Search Engines Work
The term "search engine" is often used generically to describe both true search engines and directories. They are not the same. The difference is how listings are compiled.
Search Engines Vs. Directories
Search Engines: Search engines, such as HotBot, create their listings automatically. Search engines crawl the web, then people search through what they have found.
If you change your web pages, search engines eventually find these changes, and that can affect how you are listed. Page titles, body copy and other elements all play a role.
Directories: A directory such as Yahoo depends on humans for its listings. You submit a short description to the directory for your entire site, or editors write one for sites they review. A search looks for matches only in the descriptions submitted.
Changing your web pages has no effect on your listing. Things that are useful for improving a listing with a search engine have nothing to do with improving a listing in a directory. The only exception is that a good site, with good content, might be more likely to get reviewed than a poor site.
Hybrid Search Engines: Some search engines maintain an associated directory. Being included in a search engine's directory is usually a combination of luck and quality. Sometimes you can "submit" your site for review, but there is no guarantee that it will be included. Reviewers often keep an eye on sites submitted to announcement places, then choose to add those that look appealing.
The Parts Of A Search Engine
Search engines have three major elements. First is the spider, also called the crawler. The spider visits a web page, reads it, and then follows links to other pages within the site. This is what it means when someone refers to a site being "spidered" or "crawled." The spider returns to the site on a regular basis, such as every month or two, to look for changes.
Everything the spider finds goes into the second part of a search engine, the index. The index, sometimes called the catalog, is like a giant book containing a copy of every web page that the spider finds. If a web page changes, then this book is updated new information.
Sometimes it can take a while for new pages or changes that the spider finds to be added to the index. Thus, a web page may have been "spidered" but not yet "indexed." Until it is indexed -- added to the index -- it is not available to those searching with the search engine.
Search engine software is the third part of a search engine. This is the program that sifts through the millions of pages recorded in the index to find matches to a search and rank them in order of what it believes is most relevant.
How Search Engines Rank Web Pages
Search for anything using your favorite search engine. Nearly 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 a few questions to focus the search. They also can't rely on judgment 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? They need 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 its 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 chili recipe. But cooks like to add their own secret ingredients. In the same way, search engines add spice to the location/frequency method. Nobody does it exactly the same, 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 search engine 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 unreviewed 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 penalize 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 the least by following up on complaints.
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