Understanding United States criminal law is a tremendous, potentially life-long undertaking - few can really claim they've mastered the task. Still, there are a great number of people out there who possess a useful working knowledge of the criminal law, including - in addition to lawyers, professors and law students - many regular citizens who have had occasion to deal with the criminal justice system directly. However, it seems that not nearly enough people have set themselves to the task of understanding this critical area of law, which unfortunately comes to touch almost all of our lives at least once. And we're guessing that's what brings you here.
At heart, criminal law is the tool with which society attempts to redress injuries inflicted on it by its own citizens. It is a collection of statutes, precedents and procedures which are used to restore balance in the wake of injustice - or so it's hoped.
The most distinct feature of the criminal law is that it imposes punishment on criminal offenders for their crimes. Crime, while often perpetrated against an individual, is regarded as an injury against society as a whole and so criminal punishment is meant to repay society as a whole. Civil law, by contrast, seeks to resolve disputes between individuals and organizations and - when it is appropriate - compels one side to compensate the other for losses. Whereas civil law suits are initiated by the victim, criminal law suits are initiated by the state, through a prosecutor. (Interestingly, while in the past a victim could initiate a criminal suit on behalf of the state, now a criminal prosecution must be prosecuted by the state - the victim has no say.) Defendants who are found guilty of criminal charges may be sentenced with a fine, imprisonment, or both.
Perhaps the greatest focus of the criminal law is its courts, how the courts interpret statutes and the common law to determine guilt or innocence, which punishments are meted out to redress which crimes, and how actions are criminalised in the first place. A crime is simply any action (or omission of action) that has been declared injurious by common law or statute.
The criminal law of the United States is modeled on the common law system, which is borrowed from British law. Some U.S. laws are still taken from the common law, and it is often helpful to refer to the common law for deeper understanding of a particular crime or practice. But, for the most part, U.S. criminal law is now a codified structure of federal, state and local statutes that continually amend the common law and one another. There exists a Model Penal Code to help guide the states in uniformity, however the freedom of legal legislation granted in this country means that the face of the criminal law can vary tremendously from state to state, county to county, town to town. What is a misdemeanor in one jurisdiction could be a felony in another. While some people advocate for uniformity among states and jurisdictions, others see the differences to be a great strength: in the past, the Supreme Court has demonstrated an unwillingness to impose too many uniform standards on the states, rather allowing them to be testing grounds of new techniques and applications.
In the most general terms, crimes in this country are classified as either felony or misdemeanor. Felony is a much more serious class of crime, for example, the deliberate infliction of harm on persons or valuable property, while misdemeanors are less serious offenses. Felony convictions often carry heavy fines and long incarceration in the state or federal prison systems, and misdemeanors are usually punished with lighter fines and no more than a year of incarceration in a local jail.
There are two ingredients to every crime, mens rea and actus reus. These terms are Latin for "guilty mind" and "guilty act," respectively. (An exception to this rule is found in cases of liability without fault, in which only actus reus is required to establish criminal liability.) Every defendant in the criminal law system of the United States enjoys the benefit of the Presumption of Innocence, and so, in trial, the prosecution must demonstrate "beyond a reasonable doubt" that both actus reus and mens rea were present in the defendant's actions in order to obtain a guilty verdict.