National Security Agency
Their 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 eavesdrop on the United States.
Because the agency has to keep secrets, not many people know exactly what the agency is working on.
Many people have written books about the NSA. Sometimes the books are real history books, and sometimes the books are fiction, and say things about the NSA that are not true, like in television shows like 24.