The most common visual is the pale nerd in his mother's basement who is getting into his university server to change his rivals grades to failing ones. Then there are the various Hollywood depictions which show "master criminals" manipulating traffic signals and financial markets. This is a fairly recent use of the word "hacker" and for years before it had a very different meaning.
In the early 90's when Linux (a popular free computer operating system) was introduced, the word hacker did not even exist. Users of these operating systems referred to themselves as "hackers", only due to their ability to manipulate and reuse programming code for their own purposes, outside of its originally intended purpose. If you think of them as chefs, everyone has that one basic recipe for lobster bisque, but each chef will put their own spin on the recipe to make it their own. They were / are very competent programmers that had a passion for writing their own programs.
The majority of these "hackers" used their skills for good. For example, helping a friend who needed new software to help keep track of inventory at a grocery store. Then there are some more famous hackers, including Steve Jobs and Bill Gates who made a lot of money creating a consumer computer for the home. A small percentage used their skills for less than honorable purposes, such as Kevin Poulsen and Adrian Lamo. These dishonorable hackers are what gave the noble hobby of computer manipulation its bad name.
Due to the large amount of media attention on the subject, in recent years, the term "hacker" has become synonymous with crime and people using their skills to steal and create fear. While this may be true in some instances, it is not the majority. Now we distinguish good from evil with (figurative) hats:
"White hat hacker" or "Ethical Hacker" is person who hacks for good to find their own or other organization's vulnerabilities and report them for improvement.
When the term "Black" is useed along with "Hacker" they are considered to be someone who hacks for evil maliciousness or personal gain.
"Gray hat hackers" are in that limbo status between the two who may offer to repair a vulnerability for a fee.
"Blue hat hacker" are usually outside computer security consulting firms who test software or systems for bugs looking for exploits so they can be closed prior to software or system release.
