Google has been protecting Google users from malicious web pages since 2006. We’ve built a large-scale infrastructure to automatically determine if web pages pose a risk to users. The Safe Browsing diagnostic page of a site is structured into four different categories: What is the current listing status for [the site in question] and how often a site or parts of it were listed in the past. We also provide information about the sites that were serving malicious software to users and which sites might have served as intermediaries.”]
Source: https://security.googleblog.com/2008/05/safe-browsing-diagnostic-to-rescue.html