From the preface of “Search Quality Rating Guidelines” that Google just released:
Google relies on raters, working in countries and languages around the world, to help us measure the quality of our search results, ranking, and search experience. These raters perform a variety of different kinds of “rating tasks” designed to give us information about the quality of different kinds of results in response to different kinds of queries.
The rest of the 43 page long, 713 kb, PDF document breaks down the various guidelines and criteria that the raters have to use. It is useful to read through if you want to provide content that Google and its raters woudl mark as high quality.