According to Google,
“Google Search detects spam through automated systems. Google takes action against spam, which is content that shows behavior designed to deceive users or manipulate our search systems. We issue a manual action against a site after Google human reviewers determine that its pages are engaged in spam as described in Google's Search Essentials”
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Spam Policies for Google Search
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Cloaking
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Doorways
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Hacked content
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Hidden texts and links
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Keyword stuffing
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Link Spam
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Misleading functionality
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Scraped content
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Sneaky redirects
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Spammy automatically-generated content
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Thin affiliate pages
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User-generated content
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