Google Knol was a Google project that aimed to Post the user-written articles form various topics. Most of the article related to health, medical, author opinions, sales, product review,etc. So, other users can Post their own comments to the articles. knol announced in December 13,2007 and opened the beta version in July23, 2008.
Finally, On November 22, 2011, Google announced that Knol was to be phased out in favor of Annotate (Annotum is an open-source, "open science" authoring and publishing platform for scholarly content.).
It was closed on April 30, 2012, and all content was deleted by October 1, 2012.