Link Schemes:What Google says about link schemes?

Your site’s ranking in Google search results is partly based on analysis of those sites
that link to you. The quantity, quality, and relevance of links influence your ranking.
The sites that link to you can provide context about the subject matter of your site,
and can indicate its quality and popularity. Any links intended to manipulate a site’s
ranking in Google search results may be considered part of a link scheme. This
includes any behaviour that manipulates links to your site, or outgoing links from
your site. Manipulating these links may affect the quality of our search results, and
as such is a violation of Google’s Webmaster Guidelines. The following are
examples of link schemes which can negatively impact a site’s ranking in search
results:
Some examples include:
  Buying or selling links that pass PageRank. This includes exchanging money
for links, or posts that contain links; exchanging goods or services for links; or
sending someone a “free” product in exchange for them writing about it and
including a link
  Excessive link exchanging (“Link to me and I’ll link to you”)
  Linking to web spammers or unrelated sites with the intent to manipulate
PageRank
  Building partner pages exclusively for the sake of cross-linking
  Using automated programs or services to create links to your site
  Text advertisements that pass PageRank
  Links that are inserted into articles with little coherence, for example:
  Low-quality directory or bookmark site links
  Links embedded in widgets that are distributed across various sites:
  Widely distributed links in the footers of various sites
  Forum comments with optimized links in the post or signature