Sniffers are devices that capture network packets. Their legitimate purpose is to analyze network traffic and identify purpose is to analyze network traffic and identify potential areas of concern... the "term" sniffer is derived from a product, called the Sniffer, originally manufactured by Network General Corporation.

- from Maximum Security, Second Edition, by anonymous

Note the use of legitimate in the above quote: sniffers can be remotely installed on another machine allowing a cracker to monitor network traffic to and from your machine.