A cracker is someone who breaks into someone else's computer system, often on a network; bypass password or licenses in computer programs; or in other ways intentionally breaches computers security. A cracker can be doing this for profit, maliciously, for some altruistic purpose or cause, or beacuse the challenge is there. Some breaking-and-entering has been done ostensibly to point out weaknesses in a site's security system.
The term "cracker" is not be confused with "hacker". Hackers generally deplore cracking. However, as Eric Raymond, compiler of The New Hacker's Dictionary notes, some journalists ascribes break-ins to "hackers."
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.