A cracker is someone who breaks into someone else's computer system, often on a network. A cracker can be doing this for profit, maliciously, for some altruistic purpose or cause, or because the challenge is there.
Hackers generally deplore cracking.
A classic story of the tracking down of a cracker on the Internet who was breaking into U.S. military and other computers is told in Clifford Stoll's The Cuckoo's Egg . This term was suggested by Hans Sjoholm. Inc. All rights reserved.