|Someone is wrong on|
A hacker (in the original sense) is somebody particularly skilled at computer programming. The term was popular among early computer users grouped around university computer labs in the 1960s and 1970s. The possible origin was the term "hack" at the Massachusetts Institute of Technology, meaning a well thought out and elaborate prank.
 History of the term
The term Hacker probably originated in the 1960's to describe a small community of computer programers at MIT and the Massachusetts Tech Model Railroad Club. In the 1970s the term was applied to hobbyist computer programers, including famously, Steve Jobs, Steve Wozniak and Bill Gates.
Around the time of the 1983 movie WarGames, the media seized on the term to mean young computer users who broke into computer systems illegally, and this remains the most common definition today, much to the chagrin of the older generation of computer programmers. Those who objected to the term being used for illegal activity proposed the term "cracker" for the latter, which has not caught on in the mainstream media.
"Hackers" today are a Big Scary Threat lurking out there in cyberspace, writing computer viruses, breaking into government computers, phishing for your bank account numbers, speaking a fluent 1337, and reading 2600 Magazine. Still, there are plenty of hackers in the original sense who proudly use the term, who are also a Big Scary Threat (to Microsoft and the Church of Scientology, at least) due to their advocacy of free software, open source, Linux, and strong cryptography, and their wizard-like programming skills.
 Script kiddies
A script kiddie (alternatively, click kiddie, to take into account the growing inability of the people in question even to run a script) is a 1337 h@xx0r5 that uses programs written by actual hackers to DDoS or otherwise harass people in cyberspace. When encountering a script kiddie it is important to remind them that using programs like Low Orbit Ion Cannon is hardly more challenging than using MS Paint, and therefore does not make them 1337.
 Not a hacker/hacking
Have you ever received an email from somebody that was rude, racist, profane or sexual in nature, only to have received another one from that same somebody later saying "That wasn't me, someone hacked my email?" The truth is that they weren't hacked -- rather, they left their workstation logged in while they were away from their desk, someone walked in and sent the message as a joke.
The phrase "someone hacked my account" is currently being overly - and improperly - used to describe the situation where someone at a library or Apple Store posted a status update on the Facebook profile of someone who logged in there to check messages and forgot to log out.
While in both of these cases the user violated basic acceptable use policies in leaving their accounts unprotected, the perpetrator of the jokes was not a hacker -- simply a troll in the right place at the right time. The fact that access to the accounts was already open and did not have to be "cracked" makes these instances specifically not hacking. It's like calling someone who walks in your open front door a lockpicking artist.
 Privacy concerns and security
Hacking is so widespread that year after year, governments attempt to create new laws to combat hacking, especially from other nations. While it is generally a refined mixture of bullshit and loaded language, a considerable part of it all is a serious concern. If motivated by a goal, a hacker could possibly hack into a serious system, like a bank. Even though most hackers would be content with stealing codes and movies and filling websites with thousands of litres of porn, a hacker could potentially cause mass damage if they wanted. One could, for example, hack into the system that controls traffic lights, cause water purification plants to fail, tamper with a plane's navigation system, or even hack into the cable network and have it play nothing but infomercials.
Until we get an ex-hacker in the government, we have to deal with the aroma of said bullshit and 9/11 references.
 See also
- The Jargon File