With many development carried out by Google on his algorithm in order to rank a site in the Search Engine Result Page (SERP) with more accurate, then a lot of speculation about how Google rank a site these days.
Examples of such speculation is that the backlinks and PR is not important anymore, or linking is not a factor and only On Page Optimization is the factor. But is it all true? and I think an answers that can be trusted is the answer from Google's own employees.
You can visit this page that discusses backlinks and PageRank that are answered directly by Google's employees Susan Moskwa.