The legal system in the United States is essentially the judicial branch of government. This is a comprehensive structure that incorporates state and local court systems as well, with lower-level cases being administered by state courts. Cases involving government agencies or constitutional questions are presented in federal court at the district level and appealed to the circuit when necessary.
States also utilize the district and circuit system, but local circuit court cases are specifically for civil cases involving designated amounts of money or criminal cases involving felony charges that require an indictment. Cases that are adjudicated in the state court system are normally finalized within that structure. States have intermediate courts also and use the appellate court level in the same manner as federal court structure implements the circuit level. It is important to understand that all state courts are incorporated to the U.S. Constitution by virtue of the state ratification requirement for statehood.
Federal courts are designated for interstate lawsuits and criminal cases being prosecuted by federal government policing agencies. If the primary litigants in a civil case are from different states, then the federal system is necessary because states have different laws and limited power across state lines. The federal system serves as an arbitrator with an appeals capacity for final adjudication. Criminal cases prosecuted in the federal system are adjudicated using federal statutes and federal sentencing guidelines. States use state statutes to prosecute criminals, but all precedents assessed in the federal system must be followed. Federal courts are sovereign to state courts.
The federal court system also determines the authority of other branches of government. This is the primary function of the judicial branch in the governmental system of checks and balances. Assessing constitutional violations involved in government overreach is a primary function of the federal courts. Cases involving government agencies can be appealed to the U.S. Supreme Court for ultimate adjudication if the court chooses the case.