What is PRISM?

PRISM is a electronic surveillance program operated by the United States National Security Agency (NSA) since 2007. PRISM is a government codename for a data collection effort known officially as US-984XN. It is operated under the supervision of the United States Foreign Intelligence Surveillance Court pursuant to the Foreign Intelligence Surveillance Act (FISA). 

The existence of the program was leaked by NSA contractor Edward Snowden and published by The Guardian and The Washington Post on June 6, 2013.

A document included in the leak indicated that the PRISM SIGAD was "the number one source of raw intelligence used for NSA analytic reports." The President's Daily Brief, an all-source intelligence product, cited PRISM data as a source in 1,477 items in 2012. The leaked information came to light one day after the revelation that the United States Foreign Intelligence Surveillance Court had been requiring the telecommunications company Verizon to turn over to the NSA logs tracking all of its customers' telephone calls on an ongoing daily basis.

(source: Wikipedia)