In a video answer from Google's Matt Cutts he explained that of the 700,000 messages Google sent out to webmaster via Google Webmaster Tools in the first two months of the year, only 3% of those were link related.
Matt Cutts seemed a bit upset that webmasters assumed that most of the Google Webmaster Tools notifications were link related. He set the record straight that only 3% of the 700,000 emails - that is about 25,000 emails - were link related.
Matt Cutts went on to say that 90% of those notifications are black hat related. What is black hat? He did not say but it can be anything from cloaking to white text on a white background - I'd assume.
Here is the video:
Forum discussion at Google+.