PRISM is the code name given to a top-secret surveillance program run by the NSA. PRISM was established to gather information through a variety of digital means, mainly by targeting foreign individuals through internet-based organizations. The full extent of what exactly PRISM does is not entirely known, although a series of PowerPoint slides leaked to the Washington Post and Guardian in early June 2013 provides a basic understanding. President Barack Obama and Director of National Intelligence James Clapper have both publicly confirmed the existence of such a surveillance program, though both have implied that it is narrowly tailored and constitutional.
The program allegedly involves cooperation between the NSA and nine organizations: Microsoft, Yahoo, Google, Facebook, PalTalk, Youtube, Skype, AOL and Apple. Representatives from many of these organizations, however, have denied compliance with any government surveillance efforts.
The slides indicate that the FBI gleans a variety of digital information on foreign individuals and entities, and then forwards it to the NSA to be scrutinized. In April 2013, the NSA allegedly had some 117,675 targets of surveillance in PRISM's database. Though the slides imply that efforts are made to weed out American citizens, there is no indication how PRISM guarantees that none are targeted.
PRISM's legality is based upon the Foreign Intelligence Surveillance Act (FISA).