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The term "spam", in the context of what you see in your search
results, refers to any unethical practice used to improve a page's
ranking in search engine results. Exactly what is considered spam by
search engines?

How search engines see it

• Google
  defines spam as "trying to deceive our web crawler by means of hidden
  text, deceptive cloaking or doorway pages." You can report sites you
  suspect of spam at Google's Report A Spam Result page.
• Yahoo
  defines spam as "pages (that) are created deliberately to trick the
  search engine into offering inappropriate, redundant or poor-quality
  search results." They have a pretty extensive list of what techniques
  they consider spam at their Yahoo Search Technology Content Quality Guidelines page.
• Bing
  gives a few spamming techniques "discouraged" by their webmaster
  guidelines; among them are keyword stuffing, invisible text, or false
  links.
• Ask
  defines spam as "the practice of purposely deceiving a search engine
  into returning a result that is unrelated to a user’s query, or that is
  ranked artificially high in the result set." They give quite a few
  examples of search engine spam.
  
  

While not an exhaustive list of search engines, this should give you a good idea of what is considered spam by search engines.

Lots of different Spam