Search for anything using your favorite crawler-based 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.”

OK — a librarian’s not really going to stare at you with a vacant expression. Instead, they’re going to ask you questions to better understand what you are looking for.

Unfortunately, search engines don’t have the ability to ask a few questions to focus your search, as a librarian can. They also can’t rely on judgment and past experience to rank web pages, in the way humans can.

So, how do crawler-based search engines go about determining relevancy, when confronted with hundreds of millions of web pages to sort through? They follow a set of rules, known as an algorithm. Exactly how a particular search engine’s algorithm works is a closely-kept trade secret. However, all major search engines follow the general rules below.