Search engine spam refers to pages that are considered unwanted and appear in search results with the intent to deceive or attract clicks, with little regard for relevance or overall quality of the user experience.

Some, but not all, examples include:

• Pages that harm accuracy, diversity or relevance of search results
• Pages dedicated to redirecting the user to another page (doorway pages)
• Multiple sites or pages offering substantially the same content
• Sites with numerous, unnecessary virtual hostnames
• Pages in great quantity, automatically generated or of little value (cookie cutter pages)
• Pages using methods to artificially inflate search engine ranking
• The use of text or links that are hidden from the user
• Pages that give the search engine different content than what the end-user sees (cloaking)
• Sites cross-linked excessively with other sites to inflate a site's apparent popularity (link schemes)
• Pages built primarily for the search engines or pages with excessive or off-topic keywords
• Misuse of competitor names
• Sites that use excessive pop-ups, interfering with user navigation
• Pages that seem deceptive, fraudulent or provide a poor user experience

More about Yahoo!'s content guidelines.