National Security Agency
Their stated goal is to protect the US people, by making codes and breaking codes. They collect information by "eavesdropping". This means that they listen to what people are saying in other countries, without letting the people know that they are listening. The NSA is also in charge of making sure that other countries do 'not'[source?] eavesdrop on the United States.
Because the agency keep secrets, people do not know exactly what the agency is spying on.