Let's give an example from Chris Riding's article " Page Rank Explained " (by Alexander Sadovsky's translation)

          "When Google was a tiny babe in diapers only, it was possible to tell with confidence that the reference was an exact sign of the recommendation. However things have changed at present, and there are very important reasons for that.

          The Internet has essentially changed. The reference today is rather an associated site only, the license requirement or reciprocal service (as cross references), than the true recommendation.

          As soon as you will create the search engine which considers references as recommendations, people will start trying to influence references. As soon as they begin to influence them, references will discontinue to be recommendations.

          That is what has happened. Search systems began to take references into account, and great demand for the references arose in the Internet there. Unlike the above-mentioned types of references the overwhelming majority of the established for the advancement in search systems references are not recommendations to pass to referenced page. Neither the web designer who establish the reference nor the owner of a site adduced by the reference depend seriously upon the real crossing of visitors to the site. The only purpose of such a reference is increasing of referenced site position in the search result.

          It is impossible to evaluate this phenomenon definitely. Let's remember one of the largest search engines text.

          "... We do not inform about the exhaustive list of spam methods which can cause site removal. Nevertheless here are some of them: placing of the references intended only for the increasing of relevance in search system, including participation in programs of reference exchange for the pointed purpose ..."

          Unfortunately, it is not definite here what "the reference intended only for the increasing of relevance " is. We will tell you later, how to place references for the purpose of advancement in the searcher, and you will see that it is very difficult to separate "the legal" reference-recommendation (which, nevertheless, should influence positions in search delivery) from the reference, "the relevance intended only for the increasing of relevance". Nevertheless there are some ways of reception of references-recommendations for site advancement. Such references are also entitled natural one.