An act is not automatically a crime because it is hurtful or sinful. An action constitutes a true crime only if it specifically violates a criminal statute duly enacted by Congress, a state legislature, or some other public authority. A crime, then, is an offense against the state punishable by fine, imprisonment, or death. A crime is a violation of obligations due the community as a whole and can be punished only by the state. The sanctions of imprisonment and death cannot be imposed by a civil court or in a civil action (although a fine may be a civil or a criminal penalty).

In the United States most crimes constitute sins of commission, such as aggravated assault or embezzlement; a few consist of sins of omission, such as failing to stop and render aid after a traffic accident or failing to file an income tax return. The state considers some crimes serious, such as murder and treason, and this seriousness is reflected in the corresponding punishments, such as life imprisonment or the death penalty. The state considers others crimes only mildly reprehensible, such as double parking or disturbing the peace, and consequently punishments of a light fine or a night in the local jail are akin to an official slap on the wrist.

Some crimes, such as kidnapping or rape, constitute actions that virtually all citizens consider outside the sphere of acceptable human conduct, whereas other crimes constitute actions about which opinion would be divided. For example, an 1897 Michigan statute makes it illegal to curse in front of a child, and a Nebraska law forbids bingo games at church suppers. Other criminal statutes are plainly silly: In Wisconsin it is illegal to sing in a bar, and in Louisiana it is forbidden to appear drunk at a meeting of a literary society.

The most serious crimes in the United States are felonies. In a majority of the states a felony is any offense for which the penalty may be death (in states that allow it) or imprisonment in the penitentiary (a federal or state prison); all other offenses are misdemeanors or infractions. In other states, and under federal law, a felony is an offense for which the penalty may be death or imprisonment for a year or more. Thus, felonies are distinguished in some states according to the place where the punishment occurs; in some states and according to the federal government, the length of the sentence is the key factor. Examples of felonies include murder, forcible rape, and armed robbery.

Misdemeanors are regarded as petty crimes by the state, and their punishment usually consists of confinement in a city or county jail for less than a year. Public drunkenness, small-time gambling, and vagrancy are common examples of misdemeanor offenses. Some states have a third category of offense known as infractions. Often they include minor traffic offenses, such as parking violations, and the penalty is usually a small fine. Fines may also be part of the penalty for misdemeanors and felonies.