Serial killers are individuals who have a history of multiple slayings of victims who were usually unknown to them beforehand. Their crimes are committed as a result of a compulsion that often has roots in the killer's (often dysfunctional) youth, as opposed to those who are motivated by financial gain (e.g. contract killers) or ideological/political motivations (e.g. terrorists).
http://www.washingtonpost.com/wp-dyn/metro/specials/shootings/2002/ Newspaper coverage of a series of random killings in Montgomery County and other suburbs of Washington, DC, that began October 2, 2002 by Lee Boyd Malvo and John Muhammad.