As announced on Webmaster Central blog, Google has released the full version of the Search Quality Rating Guidelines that Search Quality Raters practice to check and rate the quality of the Google search results. It is a 160-page PDF document, to understand how Search Quality Raters rate the search results they are testing.
This document had already leaked in 2008, 2001, 2012 and other times. Google had released a version back in 2013, but they have released the full version now.
If you are thinking this is the key to Google ranking algorithm. You are wrong. It is just a document help raters to rate the quality of the search results. The algorithm doesn’t relate with these guidelines. But it will help to know what Google wants to show in their search results.
As said by John Mueller on Google+
As mentioned in the blog post, these are for evaluators who assess the quality of Google’s search results – give us feedback on our experiments. Ratings from evaluators do not determine individual site rankings but are used help us understand our experiments. The evaluators base their ratings on guidelines we give them; the guidelines reflect what Google thinks search users want.
Google will keep updating this document over time.