Spiders explained

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Basically all search engine spiders function on the same principle – they crawl the Web and index pages, which are stored in a database and later use various algorithms to determine page ranking, relevancy, etc of the collected pages. While the algorithms of calculating ranking and relevancy widely differ among search engines, the way they index sites is more or less uniform and it is very important that you know what spiders are interested in and what they neglect.

Search engine spiders are robots and they do not read your pages the way a human does. Instead, they tend to see only particular stuff and are blind for many extras (Flash, JavaScript) that are intended for humans. Since spiders determine if humans will find your site, it is worth to consider what spiders like and what don't.