My SEO e-Guide by Nicolas Gremion - HTML preview

10. Document Topic

Google attempts to extract the topic of a document using the URL, low frequency words contained in the document, categorization, content analysis, clustering or summarization.

• If Google sees a significant change in the number of topics associated with a document after a stable period of set topics or the disappearance of the original topic, Google may consider that the document has been taken over as a 'doorway page' and may consider the document and any links or anchor text associated with the document as SPAM.
• Ensure your site stays consistent with its original topic.