A more intelligent surfer that probabilistically hops from page to page depending on the content of the pages and query terms the surfer that it is looking for. This model is based on a query-dependent PageRank score of a page which as the name suggests is also a function of query. When given a multiple-term query, Q={q1,q2,...}, the surfer selects a q according to some probability distribution, P(q) and uses that term to guide its behavior for a large number of steps. It then selects another term according to the distribution to determine its behavior, and so on. The resulting distribution over visited web pages is QD-PageRank.[41]
SEO techniques can be classified into two broad categories: techniques that search engine companies recommend as part of good design ("white hat"), and those techniques of which search engines do not approve ("black hat"). The search engines attempt to minimize the effect of the latter, among them spamdexing. Industry commentators have classified these methods, and the practitioners who employ them, as either white hat SEO, or black hat SEO.[49] White hats tend to produce results that last a long time, whereas black hats anticipate that their sites may eventually be banned either temporarily or permanently once the search engines discover what they are doing.[50]
I think it is important you distinguish your advice about no-following INTERNAL links and no-following EXTERNAL links for user-generated content. Most popular UGC-heavy sites have no-followed links as they can’t possibly police them editorially & want to give some indication to the search engines that the links haven’t been editorially approved, but still might provide some user benefit.
If you've read anything about or studied Search Engine Optimization, you've come across the term "backlink" at least once. For those of you new to SEO, you may be wondering what a backlink is, and why they are important. Backlinks have become so important to the scope of Search Engine Optimization, that they have become some of the main building blocks to good SEO. In this article, we will explain to you what a backlink is, why they are important, and what you can do to help gain them while avoiding getting into trouble with the Search Engines.
As Rogers pointed out in his classic paper on PageRank, the biggest takeaway for us about the eigenvector piece is that it’s a type of math that let’s you work with multiple moving parts. “We can go ahead and calculate a page’s PageRank without knowing the final value of the PR of the other pages. That seems strange but, basically, each time we run the calculation we’re getting a closer estimate of the final value. So all we need to do is remember the each value we calculate and repeat the calculations lots of times until the numbers stop changing much.”
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Backlinks are important for both search engines and end users. For the search engines, it helps them determine how authoritative and relevant your site is on the topic that you rank for. Furthermore, backlinks to your website are a signal to search engines that other external websites are endorsing your content. If many sites link to the same webpage or website, search engines can interpret that content is worth linking to, and therefore also worth ranking higher on a SERP (search engine results page). For many years, the quantity of backlinks was an indicator of a page’s popularity. But today algorithms like Google's Penguin update, were created to help with other ranking factors; pages are ranked higher based on the quality of the links that they are getting from external sites and less on quantity.

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."
These are ‘tit-for-tat’ links. For instance, you make a deal with your friend who has a business website to have him place a link to your website, and in exchange your website links back to his. In the dark ages of SEO, this used to be somewhat effective. But these days, Google considers such 'link exchanges' to be link schemes, and you may get hit with a penalty if you're excessive and obvious about it. This isn't to say that swapping links is always bad, but if your only motive is SEO, then odds are that you shouldn't do it.

Replicating competitor’s backlinks is one of the smartest ways to find new link building opportunities and improve SEO. Get started by choosing your primary competitors, the websites that are ranking on the top 5 positions for your main keywords. If they’re ranking above you, it means they have a better link profile, and they have backlinks of higher quality. Once you’ve decide which competitors to spy on, you’ll have to analyze their backlinks.
Suppose instead that page B had a link to pages C and A, page C had a link to page A, and page D had links to all three pages. Thus, upon the first iteration, page B would transfer half of its existing value, or 0.125, to page A and the other half, or 0.125, to page C. Page C would transfer all of its existing value, 0.25, to the only page it links to, A. Since D had three outbound links, it would transfer one third of its existing value, or approximately 0.083, to A. At the completion of this iteration, page A will have a PageRank of approximately 0.458.

Gaining Google's trust doesn't happen overnight. It takes time. Think about building up your relationship with anyone. The longer you know that person, the more likely that trust will solidify. So, the reasoning is, that if Google just met you, it's going to have a hard time trusting you. If you want Google to trust you, you have to get other people that Google already trusts, to vouch for you. This is also known as link-building.

9. Troubleshooting and adjustment. In your first few years as a search optimizer, you’ll almost certainly run into the same problems and challenges everyone else does; your rankings will plateau, you’ll find duplicate content on your site, and you’ll probably see significant ranking volatility. You’ll need to know how to diagnose and address these problems if you don’t want them to bring down the effectiveness of your campaign.

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