Hacker

A hacker is someone who possesses an unusually high skill in the arts of computer science and programming, and uses these skills to infiltrate computer systems, networks, and other secure platforms.

Legality
Hacking, while officially considered a criminal act in most cases (social terrorism), is not always malicious or harmful. Government agencies and large corporations may hire former hackers to help them protect their networks, intranets, and security systems. Inversely, these same organizations may employ them to hack into these areas of other organizations (corporate espionage).

The government agencies who hire former hackers usually do so in return for the exoneration of the hackers' past crimes.

Applications
Hackers are responsible for the creation of the internet, open-source software (such as Linux, and it's applications), run Usenet, and sometimes work as software troubleshooters. Those who hack phone systems and other people's computers are not considered hackers by the hacker community. They are instead called crackers. Crackers are also responsible for software cracking (removal of security features), and the creation of malware (adware, viruses, trojans, etc.).

Criminal Minds
A/V tech Penelope Garcia is officially labeled by the FBI as the world's most dangerous hacker. Her skills have served the BAU well in almost every episode. She was hired in return for exoneration of her previous illegal activities.

Real World Hackers

 * Steven Levy, the first official hacker and "coiner" of the phrase
 * Bill Gates is sometimes referred to as a hacker, as his original operating system was based on code written by Xerox in the early 80s.
 * Dennis Ritchie one of the inventors of LINUX.