There are three branches of government in the United States -- the executive branch, the legislative branch and the judicial branch. Each branch puts limits on the actions of the other branches to prevent any one person or part of the government from getting too much power. This system of *checks and balances* is central to how democracy works in the United States. The courts' role is to uphold the Constitution by striking down laws that run afoul of constitutional rights or limitations on government power.
Checks and Balances
The judiciary limits the types of laws the other branches of government can make and enforce. Courts are often tasked with deciding whether a law passed by congress or a state legislature is constitutional. For example, in McCullen vs. Coakley, the Supreme Court struck down a Massachusetts law that prevented protesters from standing on sidewalks directly in front of abortion clinics, finding that it violated the protesters' constitutional right to free speech. In Loving vs. Virginia, the Supreme Court struck down a Virginia law that made interracial marriage illegal, finding that it violated citizens' constitutional right to equal protection under the law. After a court finds a law unconstitutional, the other branches of government can no longer enforce it.
Interpreting the Law
Another important role of the judiciary is interpreting laws and applying them to individual situations because judges can't make decisions about the meaning of laws and how they should be applied in the abstract. Under Article III of the U.S. Constitution, federal courts are only authorized to decide actual cases and controversies. This means that, except in very limited circumstances, people cannot ask judges to interpret laws or tell them how the law should be applied until they are actually subjected to the law in question. For example, if a motorist doesn't understand whether a traffic law requires him to have one working tail light or two, he can't go to court to have a judge interpret the law unless he is actually ticketed for violating it. Cases come before the courts in two main ways: When a person is accused of a crime, and when there is a civil dispute between people or businesses.
A person who is arrested and accused of committing a crime is entitled to a trial before a judge and often a jury for a determination of whether she's guilty of that crime. In a criminal case, the courts' job is to make sure the constitutional rights of the accused are protected during that process. This means the judge must ensure that the accused receives a fair trial and that the government meets its burden of proving that the accused person is guilty beyond a reasonable doubt.
The courts also play a vital role in resolving civil disputes between individuals or businesses. When two businesses have a dispute over a transaction or contract, they might file a civil lawsuit asking a judge to interpret the contract and determine the parties' rights and responsibilities. Likewise, when a person claims he has been injured due to someone else's negligence or wrongdoing, he can take this claim to court where a judge decides who was responsible for the injury and whether the injured person is entitled to receive money from the wrongdoer. Civil courts also help parties resolve many other types of legal matters that don't involve crimes, such as child custody, divorce, and division of assets after a person's death.
Structure of the Court System
There are two parallel court systems in the United States -- the federal court system and state court systems. Federal courts can only hear two types of cases, those that involve violations of federal laws or constitutional rights, or disputes between citizens of two different states where more than $75,000 is at stake. State courts can hear any types of cases that arise between citizens of that particular state. Both the federal and state court systems have three levels, allowing people to appeal unfavorable decisions to at least one higher court.