Security hackers are people involved with circumvention of computer security. Among security hackers, there are several types, including:
White hats are hackers who work to keep data safe from other hackers by finding systemvulnerabilities that can be mitigated. White hats are usually employed by the target system's owner and are typically paid (sometimes quite well) for their work. Their work is not illegal because it is done with the system owner's consent.
Black hats or crackers are hackers with malicious intentions. They often steal, exploit, and sell data, and are usually motivated by personal gain. Their work is usually illegal. A cracker is like a black hat hacker, but is specifically someone who is very skilled and tries via hacking to make profits or to benefit, not just to vandalize. Crackers find exploits for system vulnerabilities and often use them to their advantage by either selling the fix to the system owner or selling the exploit to other black hat hackers, who in turn use it to steal information or gain royalties.
Grey hats include those who hack for fun or totroll. They may both fix and exploit vulnerabilities, but usually not for financial gain. Even if not malicious, their work can still be illegal, if done without the target system owner's consent, and grey hats are usually associated with black hat hackers.