One step below the Court of Appeals is the District Court. Each of the 94 districts has at least two judges; the biggest districts have 24 or more. Each district also has a U.S. bankruptcy court. District Courts are the trial courts of the federal system. Their criminal cases concern federal offenses, and their civil cases deal with matters of federal law or disputes between citizens of different states (remember subject matter jurisdiction). They’re also the only federal courts where grand juries indict the accused and juries decide the cases.
Congress determines the court districts based on size, population and case load. Some states have their own district while New York, California and Texas each have four. Judges have to live in the district they serve -- the District of Columbia is the lone exception -- but a judge may temporarily sit in another district to help with a heavy case load.
Magistrate Judges are appointed by District Judges to serve an eight-year term in a U.S. District Court. Part-time magistrates serve four-year terms. This system was started in 1968 to help District Courts with their caseloads. Both parties involved in a case have to agree to be heard by a Magistrate Judge instead of a District Judge. Magistrate Judges also conduct initial proceedings for cases such as issuing warrants, bail hearings, appointing attorneys and reviewing petitions and motions.