The United States is a representative democratic republic, which operates as a federal system. There is a federal court system, but it does not control the state court systems. There are 50 states, the District of Columbia (DC), Puerto Rico and several territories each with their own legal systems. Each state, DC, Puerto Rico and the territories have their own court systems. State court systems include many levels of courts that vary by state, counties within each state, and cities within each county.
The US is a common law system with both state and federal statutes providing civil and criminal law.
The US has a federal court system with 94 federal district courts, 13 courts of appeal and a Supreme Court. The federal system works differently in many ways than state courts. The primary difference for civil cases (as opposed to criminal cases) is the types of cases that can be heard in the federal system. Federal courts are courts of limited jurisdiction, meaning they can only hear cases authorized by the United States Constitution or federal statutes. The federal district court is the starting point for any case arising under federal statutes, the Constitution, or treaties. This type of jurisdiction is called “original jurisdiction.” Sometimes, the jurisdiction of state courts will overlap with that of federal courts, meaning that some cases can be brought in both courts. The plaintiff has the initial choice of bringing the case in state or federal court. However, if the plaintiff chooses state court, the defendant may sometimes choose to “remove” to federal court.
Cases that are entirely based on state law may be brought in federal court under the court’s “diversity jurisdiction.” Diversity jurisdiction allows a plaintiff of one state to file a lawsuit in federal court when the defendant is located in a different state. The defendant can also seek to “remove” from state court for the same reason. To bring a state law claim in federal court, all of the plaintiffs must be located in different states than all of the defendants, and the “amount in controversy” must be more than $75,000.
Each state, the District of Columbia, Puerto Rico and the Territories each has its own court system. Generally, state courts are common law https://en.wikipedia.org/wiki/Common_law courts, and apply their respective state laws and procedures to decide cases. State court systems have both trial and appellate courts. Trial courts are the courts where a case is first filed and an initial decision is made. Appellate courts review the decisions of those trial courts. Appellate courts include courts of last resort and intermediate appellate courts. Every state has a court of last resort, usually called a supreme court. Many states also have an intermediate appellate courts. Oklahoma and Texas each have two courts of last resort—one for criminal cases and one for civil cases. All but 10 states have an intermediate appellate court, often called the court of appeals. States like Alabama and Tennessee each have two intermediate appellate courts—one for criminal cases and one for civil cases.
General jurisdiction trial courts hear a wide range of civil and criminal cases. They are often called circuit, district, or superior courts. Many states have limited jurisdiction trial courts as well, to hear cases involving a specific area of law—e.g., probate, family, or juvenile law. States may also have municipal courts, county courts, or justices of the peace that hear civil cases involving a small amount of money and less serious criminal cases.