PRISM is a clandestine national security electronic surveillance program operated by the United StatesNational Security Agency(NSA) since 2007. PRISM is a government codename for a data collection effort known officially as US-984XN.

The Washington Post noted that a leaked document indicated that the PRISM SIGAD is "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 Verizonto turn over to the NSA logs tracking all of its customers' telephone calls on an ongoing daily basis.

NSA whistleblowerWilliam Binney has stated that PRISM is just another source of input of information. "The telecoms were giving NSA access to their communication lines. The Narus devices that the NSA put in different rooms around the AT&T fiber-optic network, or Verizon's network, couldn't collect everything. They could get most of it, but they couldn't get it all. So in order to get all the data, they had to go to the service providers to fill in the blanks. That's what the PRISM program is for-to fill in the blanks. It also gives the FBI basis for introducing evidence into court."

(en.wikipedia.org)