@rtretheweyAug 28.2012 — #If you're asking how PageRank is calculated, it's still based on links. The actual calculation has evolved the original formula published by Google's founders, of course. And while nobody outside the company really knows the details, it seems clear that they give more weight to links in body text than in footers, blogrolls, and other less content-related areas of a page, and some very low quality pages seem to pass no PageRank at all. From there, you can start to talk about more esoteric factors like 'trust' or 'authority', but it all comes back to links when you're talking about PageRank itself.
If you're asking how websites obtain the data, they all rely on the online data used by the PageRank display in the Google Toolbar. There is no other publicly-available source.