In the world of search engine optimization, we all know the value of back-links, these can be better defined as votes of trust which point back to our website. There are several types of links and they are ranked differently by the search engines, a few are: one-way links, reciprocal links, three-way links, etc.
The most important type of links are the one-way links since reciprocal links have somewhat lost their value due to the incredible link spamming which took place a few years ago.
The reason many reciprocal links lost value is because they were mostly placed in what were called "link pages", these pages contained huge numbers of links and many times they pointed to websites which were completely outside of the linking site's niche; in other words pharmacy websites were linking to real estate sites, sports sites were linking to technology sites and so on. As you can see, this linking structure offered no benefit for the visitor and was specifically designed to manipulate the search engines.
Google quickly adapted to these changes and reciprocals were downgraded, this gave one-way links more value as far as SEO. These type of links are hard to get since webmasters need to write about a specific source and quote it on their sites by providing a link to it, this represented a serious problem to webmasters who heavily relied on automated reciprocal link acquisition.
Link exchange networks which operate even at this point in time, made things easier for the search engines to detect the linking patterns and anchor text utilization, this last factor helped determine if the reciprocals were in fact spam or if they were good links.
By using link networks to spam the search engines, site owners have only one way to specify anchor texts, meaning if the keyword they want to rank for is "search engine optimization" they would have to enter that term in the automated system and that would be the anchor texts linked to their sites from the thousand of sites participating in the network.
By running a simple back-link analysis on such sites it is easy to see that they have participated in networks designed to manipulate the search engine result pages since their main term comes up repeatedly hundreds of thousands of times.
At this point you may be wondering "Well, how can we rank for a specific keyword without looking spammy?", the answer is simple, you can vary the anchor text, use juxtaposition or even make long tail phrases from them. For instance, if the term you are trying to rank for is "apple pie" you can use the following anchor text variations to rank for long tail keywords as well as for the main term: "delicious apple pies", "home made apple pies", "gourmet apple pies", "apple pie making secrets" and so on.
As you see the main keyword is mentioned in the anchor phrases but they are not redundant which works a lot better in terms of getting better rank and traffic from the search engines, especially from Google.