A cracker is a type of computer hobbyist who enjoys trying to gain unauthorized access to computer systems. That means they are a kind of low-tier hacker, not out to do damage or steal information, merely to prove their own skills. They often leave a sort of calling card to prove their success either to themselves or within their community. While many crackers are not out to harm the sites they crack, some definitely are, and they pose a security risk for their targets.
