PageRank is only a fraction of Google Ranking (SERP)

Relevancy is determined by over 200 factors, one of which is the PageRank for a given page. PageRank is the measure of the importance of a page based on the incoming links from other pages. In simple terms, each link to a page on your site from another site adds to your site’s PageRank.

Google PageRank source

Further reading on PageRank

Q: My site’s PageRank has gone up / gone down / not changed in months!
A:
 Don’t worry. In fact, don’t bother thinking about it. We only update the PageRank displayed in Google Toolbar a few times a year; this is our respectful hint for you to worry less about PageRank, which is just one of over 200 signals that can affect how your site is crawled, indexed and ranked. PageRank is an easy metric to focus on, but just because it’s easy doesn’t mean it’s useful for you as a site owner. If you’re looking for metrics, we’d encourage you to check out Analytics, think about conversion rates, ROI (return on investment), relevancy, or other metrics that actually correlate to meaningful gains for your website or business.

Google PageRank FAQ Source

Matt Cutts answers: How can I identify causes of a PageRank drop?

“Please be wary if someone approaches you and wants to pay you for links or “advertorial” pages on your site that pass PageRank. Selling links (or entire advertorial pages with embedded links) that pass PageRank violates our quality guidelines…”

Source: A reminder about selling links that pass PageRank

Google Webmaster Guidelines

Webmaster Guidelines – Best practices to help Google find, crawl, and index your site