A crime is an act that is in violation of a criminal law. An act that merely violates a civil law is not a crime, although it may be a tort. It is possible for an act to be covered by both criminal and civil laws; an example is assault. An act in violation of God's law is called a sin.
Crimes are committed by criminals. They can be individuals, groups, corporations, or governments. They can be fought by individuals (taking the law into one's own hands - which in some instances can be a crime in itself), militias, mob rule, or by the police.
Where crime has been committed, a criminal may be identified, accused, and taken to a court of law. Courts require evidence in order to proceed to the matter to a trial. If the court is convinced that the accused criminal has committed the crime, it may judge the criminal guilty of the crime and issue a punishment. This may include restitution, fines, community service, a prison sentence, or death. A crime punishable by death is known as a capital crime.
Over time a given act can be legal, then declared a crime (criminalization), then made legal again (decriminalization). One example is the consumption of alcohol in the context of Prohibition. Throughout the world, debates exist over the criminal status of various activities, such as prostitution and marijuana cultivation and use.
The academic study of crime is known as criminology. The preponderance of crime is measured as the crime rate, and usually expressed as a ratio (13 homicides per 10000 people, for example). Different crimes can also be grouped into categories, such as violent crime. Informal categories include white-collar crime and victimless crime.