Although it may not technically be considered "web spam," the engines all have methods to determine if a page provides unique content and "value" to its searchers before including it in their web indices and search results. The most commonly filtered types of pages are "thin" affiliate content, duplicate content, and dynamically generated content pages that provide very little unique text or value. The engines are against including these pages and use a variety of content and link analysis algorithms to filter out "low value" pages from appearing in the results.
Google's 2011 Panda update took the most aggressive steps ever seen in reducing low quality content across the web, and Google continues to update this process
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