Website owners recognized the value of a high ranking and visibility in search engine results,[6] creating an opportunity for both white hat and black hat SEO practitioners. According to industry analyst Danny Sullivan, the phrase "search engine optimization" probably came into use in 1997. Sullivan credits Bruce Clay as one of the first people to popularize the term.[7] On May 2, 2007,[8] Jason Gambert attempted to trademark the term SEO by convincing the Trademark Office in Arizona[9] that SEO is a "process" involving manipulation of keywords and not a "marketing service."
The PageRank formula also contains a damping factor (d) . According to the PageRank theory, there is an imaginary surfer who is randomly clicking on links, and at some point he gets bored and eventually stops clicking. The probability that the person will continue clicking at any step is a damping factor. Thus, this factor is introduced to stop some pages having too much influence. As a result, their total vote is damped down by multiplying it by 0.85 (a generally assumed value).
So, as you build a link, ask yourself, "am I doing this for the sake of my customer or as a normal marketing function?" If not, and you're buying a link, spamming blog comments, posting low-quality articles and whatnot, you risk Google penalizing you for your behavior. This could be as subtle as a drop in search ranking, or as harsh as a manual action, getting you removed from the search results altogether! 

One final note is that if the links are not directly related to the subject, or you have no control over them, such as commentors’ website links, maybe you should consider putting them on another page, which links to your main content. That way you don’t leak page rank, and still gain hits from search results from the content of the comments. I may be missing something but this seems to mean that you can have your cake and eat it, and I don’t even think it is gaming the system or against the spirit of it. You might even gain a small sprinkling of page rank if the comment page accumulates any of it’s own.
There are ten essential types of marketing that can be done online. Some of these can be broken down into organic marketing and others can be categorized as paid marketing. Organic, of course, is the allure of marketing professionals from around the planet. It's free and its unencumbered traffic that simply keeps coming. Paid marketing, on the other hand, is still a very attractive proposition as long as the marketing pays for itself by having the right type of offer that converts.
Make it as easy as possible for users to go from general content to the more specific content they want on your site. Add navigation pages when it makes sense and effectively work these into your internal link structure. Make sure all of the pages on your site are reachable through links, and that they don't require an internal "search" functionality to be found. Link to related pages, where appropriate, to allow users to discover similar content. Denver Page Rank Click Here