Someone who specializes in finding exploits in computer systems. There main goal is to gain unauthorized access to networks and systems, usually in order to gain further information about the network. They are technical experts and usually highly advanced programmers.
There are 2 types of hackers: black hats and white hats. Black hats are the 'bad guys' while white hats are the 'good guys'. Black hats hack for there own needs, such as stealing money from someones credit card over the internet. White hats get paid to find exploits in programs and websites and then tell the company about them so they can fix them.
A hacker broke into a federal bank and stole $50,000.