The classic serial killer takes extended breaks between slayings, ranging from weeks to months. Victims usually fit a particular profile, such as prostitutes or elderly women; the former are a favorite target, as their disappearances seldom provoke much alarm from police. Serial killer Jeffrey Dahmer, for example, preyed chiefly upon gay hustlers, a community that existed on Milwaukee's fringes. Sex somehow plays into the bulk of serial killings. Victims are often molested, or selected because they strike a dark Freudian chord with the murderer.
Spree killers, on the other hand, pack their mayhem into a brief time span—killing becomes a full-time job, so to speak. The spree is often precipitated by a specific rage-inducing event, such as a romantic breakup or family spat. Loved ones are typically the initial victims; the killer then goes on the lam, slaughtering people along the way without much forethought. These victims do not fit any discernible profile but may be dispatched out of necessity—say, because the killer needs their car. Because of the high-profile, indiscriminate nature of the killings, police attention is intense, and the killer is usually apprehended (or commits suicide) after a few weeks. Andrew Cunanan, who chalked up five murders during a 1997 cross-country trek, is often pointed to as the prototypical spree killer.
The Washington sniper doesn't quite conform to this taxonomy of evil. The frequency of the killings and randomness of the victims suggests that he's a standard spree killer. But he is also confining his activity to one geographic area and premeditating the slayings, two peculiarities that jibe more closely with the serial-killer model.