How does Google judge quality content ?

Everybody talks about the need to provide quality content on your site if you want to rank well in searches. But how do search engines identify quality content ?

Successive Google algorithm updates (culminating in the recent Panda 4.1) aim to refine results so that they match the intent of the search query and deliver the most comprehensive, accessible and well-written answer.

Put simply, Google (and other search engines) are always looking for genuine quality content, and are increasingly smart at finding (and dismissing) pages that try to game the system.