What is the Paris climate change deal and what is the impact of the US withdrawal?

Professor Lord Stern was interviewed by ITV news in reaction to President Trump’s announcement that the US will withdraw from the Paris agreement (and negotiate re-entering the agreement).

http://www.itv.com/news/2017-06-01/what-is-the-paris-climate-change-agreement-and-what-would-be-the-impact-of-a-us-withdrawal/