A federal prosecutor, also known as a United States Attorney, works for the US government and prosecutes or defends court cases on its behalf. They operate as part of the US Department of Justice and try cases in the federal district courts and courts of appeal. There is one federal prosecutor assigned to each judicial court district throughout the US and its territories. The federal prosecutor in each district manages an office staffed with hundreds of assistant US attorneys.
Each judicial district in the US has a US Attorney's Office that is run by a federal prosecutor. The prosecutor is the head lawyer representing the government's interests in criminal and civil cases in that jurisdiction. He manages a staff of subordinate attorneys and support personnel, but does not try every case alone, or even most of the cases.
Instead, the federal prosecutor assigns cases to the assistant attorneys that work for that office. One of his main duties is to manage this allocation of work so that the right attorneys are assigned to cases they can win. A prosecutor's work performance is typically judged by the office's success rate in wining cases.
A federal prosecutor does try certain high profile cases himself, however. The US Attorney's Office represents the government by prosecuting defendants in criminal cases that deal with violations of federal criminal law and by either prosecuting on behalf of or defending the government in civil cases. For example, he may act as the prosecutor in a kidnapping case where the suspect took the victim across state lines, since that is a violation of federal law. Likewise, the federal prosecutor might defend the government in a civil case in which a citizen sues for racial discrimination by a government agency. He may also prosecute a civil case on behalf of the government in a suit against a state to return a federal budget allocation.
In addition to his administrative duties and trying cases in court, the US prosecutor is also the face of justice in his jurisdiction. He is responsible for interacting with the media and the public and interfacing with other law enforcement agencies to prosecute cases. The federal prosecutor must also work with his counterparts and superiors in the US Justice Department toward common goals. Ultimately, he is responsible for carrying out the mandates of the US Attorney General, who is the chief attorney for the whole country.