Crimes may be broadly classified or categorized by their subject matter or nature—such as crimes against the person (assault, battery, murder) or crimes against property (criminal trespassing, burglary, robbery, bribery, theft, arson). But the more practical and relevant classification of crimes is by the seriousness or severity of the crime and the related punishment. There are generally four such classifications of crimes: (1) felonies; (2) misdemeanors; (3) felony-misdemeanors; and (4) infractions.

Felonies are the most serious crimes—usually due to the malicious intent required (intent to kill), or the severe result of the crime (loss of life, serious injury, destruction or theft of valuable property)—and include murder, rape, aggravated assault, robbery involving the threat of deadly force (gun, knife), and theft or embezzlement of large amounts of money. In many states and the federal system (jurisdictions) there are different degrees of felonies, depending on the severity of the crime. Depending on the jurisdiction and the specific crime, the punishment for a felony conviction can range from probation to home confinement to the death penalty. Other consequences of a felony conviction include the inability to vote, own a gun, or be eligible for hire in many jobs or careers.

Misdemeanors are less serious crimes than felonies—usually because there is a less malicious intent required to commit the crime, or because the result of the misdemeanor crime is less serious than the result of a felony crime. Misdemeanors are usually punishable by jail time of one year or less, and some misdemeanors are not punishable by any jail time. Misdemeanors may also be punishable by a fine, probation, participation in a rehabilitation program, or community service. Misdemeanors are generally not punishable by prison time. Although jail and prison both include confinement and loss of freedom, there are significant differences in these two systems. Jails are usually operated by cities, counties, and states, and are often less restrictive in their confinement because they house people who have committed less serious crimes. Prisons usually house people who have committed federal crimes, and sometimes more serious state crimes.

Felony-misdemeanors (also known as wobblers) are crimes that the government may choose to prosecute and punish as a felony or as a misdemeanor—depending on the nature and severity of the circumstances of the alleged crime. Although the prosecutor may charge and prosecute such crimes as a felony or as a misdemeanor, the judge generally makes the decision on how the crime will be punished (as a misdemeanor or as a felony), and isn’t constrained by how the prosecutor chose to prosecute the case.

Infractions (also known as violations) are the least serious crimes, and include minor offenses such as motor vehicle offenses (illegal parking, speeding, no turn signal) and jaywalking. Infractions are usually punishable by a fine or completion of a traffic school or defensive driving course.

In Montana, crimes are classified based on their severity, with felonies being the most serious, followed by misdemeanors, felony-misdemeanors, and infractions. Felonies in Montana include crimes such as murder, rape, and robbery, and can result in severe punishments ranging from probation to the death penalty. Convicted felons may also face collateral consequences like losing the right to vote or possess firearms. Misdemeanors are less serious offenses, often resulting in less than one year of jail time, fines, or alternative sentences like probation or community service. Montana does not typically incarcerate individuals convicted of misdemeanors in prison, reserving such facilities for more serious offenses. Felony-misdemeanors, or 'wobblers,' can be prosecuted either as felonies or misdemeanors, with the final sentencing decision often resting with the judge, who is not bound by the prosecutor's initial charge. Infractions are minor violations, such as traffic offenses, and are generally punishable by fines or educational courses rather than jail time.