The probability for the random surfer not stopping to click on links is given by the damping factor d, which is, depending on the degree of probability therefore, set between 0 and 1. The higher d is, the more likely will the random surfer keep clicking links. Since the surfer jumps to another page at random after he stopped clicking links, the probability therefore is implemented as a constant (1-d) into the algorithm. Regardless of inbound links, the probability for the random surfer jumping to a page is always (1-d), so a page has always a minimum PageRank.
Something a lot of people seem to have overlooked was hinted at in Greg Boser’s comment above. Greg identified that there is a major (and unfair) disparity with how authority sites such as Wikipedia disrupt the linkscape by run-of-site nofollows. Once Wikipedia implemented the no-follows, previously high-value links from Wikipedia were rendered worthless making the site less of a target for spammers. Increasingly large sites are following suit in order to cleanse their own pages of spam.
Moreover, the PageRank mechanism is entirely general, so it can applied to any graph or network in any field. Currently, the PR formula is used in bibliometrics, social and information network analysis, and for link prediction and recommendation. It's even used for system analysis of road networks, as well as biology, chemistry, neuroscience, and physics.
(1 - d) - The (1 – d) bit at the beginning is a bit of probability math magic so the “sum of all web pages' PageRanks will be one”: it adds in the bit lost by the d(.... It also means that if a page has no links to it (no backlinks) even then it will still get a small PR of 0.15 (i.e. 1 – 0.85). (Aside: the Google paper says “the sum of all pages” but they mean the “the normalised sum” – otherwise known as “the average” to you and me.
SEM, on the other hand, costs money but can deliver very rapid results. Your website must be optimized to make sales or at least drive a customer to get in touch (GIT – in marketing terms) so you can make a sale. You should approach SEM with care and make sure you completely understand how much money you have exposed at any one time. Start slow and evaluate your results.
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.