Because different levels of government in the U.S. operate independently of one another, there are many differences between persons who perform this function at the federal, state, and county levels. The proper title for an appointed federal prosecutor at the local level (as opposed to an appointed U.S. Department of Justice prosecutor based in Washington, DC) is United States Attorney. Such officers are appointed by the President of the United States, serve under the Attorney General, and prosecute cases in the district courts of the federal government. United States Attorneys, in turn, hire prosecutors to handle the daily affairs of the office; they are known as Assistant United States Attorneys, or AUSAs.
Most states also have a district attorney who oversees prosecutions throughout the state. A district attorney of a state is occasionally informally referred to as the state's attorney. Care should be taken to not confuse the two.
In the United Kingdom positions equivalent to a District Attorney are the Crown Prosecutor in England, Wales and Northern Ireland, and the Procurator Fiscal in Scotland. Many Commonwealth countries use Crown Prosecutor, while Canada uses the titles of Crown Attorney or Crown Counsel.