Internet service providers in many countries are legally required (e.g., via Communications Assistance for Law Enforcement Act (CALEA) in the U.S.) to allow law enforcement agencies to monitor some or all of the information transmitted by the ISP. Furthermore, in some countries ISPs are subject to monitoring by intelligence agencies. In the U.S., a controversial National Security Agency program known as PRISM provides for broad monitoring of Internet users traffic and has raised concerns about potential violation of the privacy protections in the Fourth Amendment to the United States Constitution. Modern ISPs integrate a wide array of surveillance and packet sniffing equipment into their networks, which then feeds the data to law-enforcement/intelligence networks (such as DCSNet in the United States, or SORM in Russia) allowing monitoring of Internet traffic in real time.
What does PRISM do? 
provides for broad monitoring of Internet users traffic