Spiders, also known as crawlers, are robot programs that have a set of rules called algorithms which guide them as they look through documents on the Internet.  For example, some follow every link on every home page.  Other spiders ignore some items and look specifically for the most popular home pages.  Some spiders do not index PDF files, audio files, graphic files, etc., only HTML files.   Because each one is different, you do not get exactly the same results for the same search using different search engines.

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