A hacker is a person who creates and modifies computer software and computer hardware, including computer programming, administration, and security-related items. The term usually bears strong connotations, but may be either favorable or denigrating depending on cultural context (see the Hacker definition controversy). Common definitions include:

1. In computer programming, a hacker is a programmer who hacks or reaches a goal by employing a series of modifications to exploit or extend existing code or resources. For some, "hacker" has a negative connotation and refers to a person who "hacks" or uses kludges to accomplish programming tasks that are ugly, inelegant, and inefficient. This negative form of the noun "hack" is even used among users of the positive sense of "hacker".

2. In computer security, a hacker is a person who specializes in work with the security mechanisms for computer and network systems. While including those who endeavor to strengthen such mechanisms, it more often is used, especially in the mass media, to refer to those who seek access despite them.

3. In other technical fields, hacker is extended to mean a person who makes things work beyond perceived limits through their own technical skill, such as a hardware hacker, or reality hacker.

4. In hacker culture, a hacker is a person who has attained a certain social status and is recognized among members of the culture for commitment to the culture's values and a certain amount of technical knowledge.

Categories of hacker:

The hacker community, the set of people who would describe themselves as hackers or described by others as hackers, falls into at least four partially overlapping categories. Sometimes alternate terms such as "cracker" are used in an attempt to more exactly distinguish which category of hacker is intended, or when attempting to put a contextual distance between the categories due to the Hacker definition controversy.

Hacker: Highly skilled programmer

The positive usage of hacker is one who knows a (sometimes specified) set of programming interfaces well enough to program rapidly and expertly. This type of hacker is well-respected (although the term still carries some of the meaning of hack), and is capable of developing programs without adequate planning or where pre-planning is difficult or impossible to achieve. This zugzwang gives freedom and the ability to be creative against methodical careful progress. At their best, hackers can be very productive. The technical downside of hacker productivity is often in maintainability, documentation, and completion. Very talented hackers may become bored with a project once they have figured out all of the hard parts, and be unwilling to finish off the "details". This attitude can cause friction in environments where other programmers are expected to pick up the half finished work, decipher the structures and ideas, and bullet-proof the code. In other cases, where a hacker is willing to maintain their own code, a company may be unable to find anyone else who is capable or willing to dig through code to maintain the program if the original programmer moves on to a new job.

Additionally, there is sometimes a social downside associated with hacking. The stereotype of a hacker as having gained technical ability at a cost in social ability has historical basis in an uncomfortable amount of factual foundation in many individuals. While not universal, nor even restricted to hackers, the difficulty in relating to others and the often abrasive personalities of some hackers makes some of them difficult to work with or to organize into teams. On the other hand, it is not uncommon for hackers to thrive on social interaction.

Hacker: Computer and network security expert

In the networking sense, a hacker is one who specializes in work with the access control mechanisms for computer and network systems. This includes individuals who work toward maintaining and improving the integrity of such mechanisms. However, the most common usage of hacker in this respect refers to someone who exploits systems or gains unauthorized access by means of clever tactics and detailed knowledge, while taking advantage of any carelessness or ignorance on the part of system operators. This use of hacker as intruder (frequent in the media) generally has a strong negative connotation, and is disparaged and discouraged within the computer community, resulting in the modern Hacker definition controversy.

For such hackers specializing in intrusion, the highly derogatory term script kiddies is often used to indicate those who either claim to have far more skill than they actually have, or who exclusively use programs developed by others to achieve a successful security exploit.

Hacker: Hardware modifier

Another type of hacker is one who creates novel hardware modifications. At the most basic end of this spectrum are those who make frequent changes to the hardware in their computers using standard components, or make semi-cosmetic themed modifications to the appearance of the machine. This type of Hacker modifes his/her computer for performance needs and/or aesthetics. These changes often include adding memory, storage or LEDs and cold cathode tubes for light effects. These people often show off their talents in contests, and many enjoy LAN parties. At the more advanced end of the hardware hackers are those who modify hardware (not limited to computers) to expand capabilities; this group blurs into the culture of hobbyist inventors and professional electronics engineering. An example of such modification includes the addition of TCP/IP Internet capabilities to a number of vending machines and coffee makers during the late 1980s and early 1990s.

Hackers who have the ability to write circuit-level code, device drivers, firmware, low-level networking, (and even more impressively, using these techniques to make devices do things outside of their spec sheets), are typically in very high regard among hacker communities. This is primarily due to the enormous difficulty, complexity and specialized domain knowledge required for this type of work, as well as the electrical engineering expertise that plays a large role. Such hackers are rare, and almost always considered to be wizards or gurus of a very high degree.

Hacker stereotypes

There are theoretical types of hackers who are considered to possess an atypical level of skill beyond that of other meanings of the positive form of "hacker", which include the Guru and the Wizard.

In some portions of the computer community, a Wizard is one who can do anything a hacker can, but elegantly; while a Guru not only can do so elegantly, but instruct those who do not know how. In other sub-communities, a Guru is one with a very broad degree of expertise, while a Wizard is expert in a very narrow field. In practice, such exact distinction are usually more at home in a RPG world, and not often heard in actual conversation.

Within the mainstream media, hackers are often characterised as strange, mysterious, reclusive, and especially tricky. This may be seen as an extension of the human tendency to stigmatise what is ill-understood, which used often to be applied to natural philosophers who were often thought by superstitious neighbours to be wizards or mystics. One such example was Leonardo da Vinci, who was thought to be a necromancer due to his extensive (and extraordinary at that time) knowledge of human anatomy and his study of dead bodies.