Technically PRISM is not a program, it's an internal government computer system used by a multitude of government agencies. However, since the startling revelations by Edward Snowden in June of 2013, PRISM is now destined to be colloquially defined as a secret government surveillance program.
The PRISM Story Breaks
On June 6, 2013 the Washington Post and Guardian reported the NSA had granted themselves unfettered access to millions of users private data using a surveillance program, code-named PRISM. According to the original reporting, the NSA and FBI has "direct access to the systems of Google, Facebook, Apple and other US internet giants" and that these companies "participate knowingly" in a widespread surveillance program.
Also a top-secret court order, issued to Verizon, was leaked. The order demands Verizon give the FBI "all call detail records ...(i) between the United States and abroad; or (ii) wholly within the United States, including local telephone calls."
The Original PRISM Stories Turn Out to Be Slightly Salacious
The tech giants of Silicon Valley all deny being involved in any such surveillance program or even being aware of such a program. They all claim the government has no direct access to their user data, and that they only provide the government data to comply with specific court orders, as is their legal requirement. The government also denies having direct access to private company servers.
The government does confirm they engage in intelligence gathering that invloves court orders to communications companies to produce specific data. They say this is legal under Section 702 of the Foreign Intelligence Surveillance Act (FISA) that allows them to scrutinize targeted foreign communication.
The Director of National Intelligence, James R. Clapper, goes further and declassifies some material to explain that PRISM is not a code name but an acronym for (Planning tool for Resource Integration, Synchronization, and Management) He says that PRISM can only be used with FISA warrants to monitor foreign communication that passes through U.S. based servers (Much of the Internet infrastructure is located inside the United States -- so a communication from a terrorist in Pakistan to a terrorist in Yemen is likely to end up as data that travels through U.S. based servers, routers, and nodes.). The Washington Post edits their original story to remove some of the more salacious claims like "direct access".
Are They Spying on Me or Not?!
Probably not. The NSA's PRISM program can't legally be used to intentionally target any person in the United States, nor can PRISM be used to target a person outside the United States if the purpose is to acquire information from a person inside the United States.
What is the Big Deal?
Privacy and The Fourth Amendment. The government technically could spy on you, if they wanted. The world now has proof that the U.S. government can monitor almost any communication in real time - they have the technical ability to watch your thoughts form almost as quickly as you type them. This complete Big Brother capability can no longer simply be dismissed as the purview of conspiracy theorists or the plot line for bad political thrillers. The government claims that analysis is only done on the data when they have a legal court order. However, many people are extremely upset that their data is available to the government even if the government claims the data is not analyzed without cause.
Has PRISM Saved Lives?
It depends on whether you believe the NSA or not. On June 18th, 2013, NSA Director Keith Alexander testified that the PRISM program helped avert more than 50 “potential terrorist events.”