The query interface and search results pages truly are the only parts of a search engine that the user ever sees. Every other part of the search engine is behind the scenes, out of view of the people who use it every day. That doesn’t mean it’s not important, however. In fact, what’s in the back end is the most important part of the search engine, and it’s what determines how you show up in the front end.

If you’ve spent any time on the Internet, you may have heard a little about spiders, crawlers, and robots. These little creatures are programs that literally crawl around the Web, cataloging data so that it can be searched. In the most basic sense, all three programs — crawlers, spiders, and robots — are essentially the same. They all collect information about each and every web URL.

This information is then cataloged according to the URL at which they’re located and are stored in a database. Then, when a user uses a search engine to locate something on the Web, the references in the database are searched and the search results are returned.

Databases

Every search engine contains or is connected to a system of databases where data about each URL on the Web (collected by crawlers, spiders, or robots) is stored. These databases are massive storage areas that contain multiple data points about each URL.
The data might be arranged in any number of different ways and is ranked according to a method of ranking and retrieval that is usually proprietary to the company that owns the search engine.
You’ve probably heard of the method of ranking called PageRank (for Google) or even the more generic term quality scoring. This ranking or scoring determination is one of the most complex and secretive parts of SEO. How those scores are derived, exactly, is a closely guarded secret, in part because search engine companies change the weight of the elements used to arrive at the score according to usage patterns on the Web.
The idea is to score pages based on the quality that site visitors derive from the page, not on how well web site designers can manipulate the elements that make up the quality score. For example, there was a time when the keywords that were used to rank a page were one of the most important factors in obtaining a high-quality score.