Below is a quote straight out of Google Webmaster Tools:
However, some SEOs and webmasters engage in the practice of buying and selling links that pass PageRank, disregarding the quality of the links, the sources, and the long-term impact it will have on their sites. Buying or selling links that pass PageRank is in violation of Google's webmaster guidelines and can negatively impact a site's ranking in search results.
What is a LINK that PASSES PAGERANK?
However, some SEOs and webmasters engage in the practice of buying and selling links that pass PageRank, disregarding the quality of the links, the sources, and the long-term impact it will have on their sites. Buying or selling links that pass PageRank is in violation of Google's webmaster guidelines and can negatively impact a site's ranking in search results.
What is a LINK that PASSES PAGERANK?