how google earns money...

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Is it true that the sites which appears on google when we search something(eg by giving the keyword) are paying to google to link up their site,so that people can get into or see their site?another thing i have read is google have some sort of mechanism called spider which search for the related link when you give the keyword...and the spider will rank your site or the crawled sites on some aspects(ie youe tag name,how you organize your site and so on...).so which of this is true and if the second is true will the site have to pay for google?