The National Security Agency is an intelligence-gathering agency within the U.S. Department of Defense. It is one of the most secretive agencies within the U.S. government. For many years, even its existence was rarely acknowledged by the government, and its budget is still a secret.
The NSA has two main divisions, the Signals Intelligence Directorate (SID) and the Information Assurance Directorate (IAD). SID gathers, analyzes and disseminates intelligence information from foreign signals. IAD’s mission is to prevent foreign adversaries from accessing classified information. Basically, the NSA eavesdrops on all foreign communications and encodes all U.S. government communications.
For more information about the NSA, view the following web page: http://en.wikipedia.org/wiki/National_Security_Agency