Black Hat Hackers/Crackers
Often referred to as hackers, a term which also applies to people doing good work on computers, crackers are people who break into computer systems using specialized knowledge and tools.
While often doing this for no other reason than to create mischief, some crackers do break into systems to steal data or to commit other crimes. Crackers take advantage of any insecurity in a system to gain a foothold, including social engineering.
Crackers vary greatly in skill level; neophyte crackers are easily thwarted while the most skilled will, if determined, break into even the most protected systems that are connected to the Internet.