Multiple murderers are people who have killed more than one victim. Based on the patterns of their murders, multiple killers are classified into three basic categories -- mass murderers, spree killers, and serial killers. Here is the definition of each category of multiple killers:

Mass Murderers: A mass murderer kills four or more people at one location during one continuous period of time, whether it is a few minutes or over a period of days.

An example of a mass murderer would be Richard Speck, who killed eight student nurses in July of 1966, in a single night in their south Chicago townhouse. Killers who murder several members of their family, also fall into the mass murderer category.

Spree Killers: These murderers kill two or more victims, but are more than one location. Although their murders occur in separate locations, their spree is considered a single event, because there is no "cooling off" period between the murders. Robert Polin is an example of a spree killer. In October 1975 he killed one student and wounded five others at an Ottawa high school after earlier raping and stabbing a 17-year-old friend to death.

Serial Killers: Serial killers murder three or more victims, but each is killed on separate occasions. Unlike mass murderers and spree killers, serial killers usually select their victims, have cooling off periods between murders, and plan their crimes carefully. Some serial killers travel widely to find their victims, such as Ted Bundy, but others remain in the same general geographic area.

In 1988, Ronald Holmes, a criminologist at the University of Louisville who specializes in the study of serial killers, identified four subtypes of serial killers.

The Visionary -- Usually psychotic, the visionary is compelled to murder because he hears voices or sees visions ordering him to kill certain kinds of people.

Mission-Oriented -- Targets a specific group of people who he believes are unworthy to live and without whom the world would be a better place.

Hedonistic Killer -- Kills for the thrill of it because they enjoy the act of killing and sometimes becomes sexually aroused during the act of murder.

Power-Oriented -- Kills to exert ultimate control over his victims. These murderers are not psychotic, but they are obsessed with capturing and controlling their victims and forcing them to obey their every command.