Ford v. Wainwright

Manslaughter is popularly--and erroneously--thought of as a form of murder. In terms of its ordinary definition, of course, manslaughter is indeed a type of killing, but legally it is not murder. For an act to qualify as manslaughter in legal terms, a killing may be intentional, but it cannot be premeditated or a product of malicious aforethought.

Thus manslaughter is to be distinguished from a situation in which a person causes death, but is not guilty of intent, negligence, or other wrongdoing. Examples of these situations would be self-defense, or pure accidents, as when a driver obeying all traffic laws hits and kills someone who runs in front of his car on a dark night. Killing in such circumstances is usually not prosecuted; manslaughter, by contrast, is clearly a crime.

Voluntary manslaughter occurs when someone intentionally (but without premeditation) kills another.

Involuntary manslaughter, by contrast, happens without intent, and is divided into two categories, criminal-negligence manslaughter and unlawful-act manslaughter. To use the situation of the driver referred to above, if he were asleep at the wheel when he killed the person running across the road, this would probably be criminal negligence; if, on the other hand, he were drunk, that would most likely qualify as unlawful-act manslaughter.