If you don't know exactly what a hacker is, join the club. Many people don't understand the term or its true implications. The meaning has changed a bit over the years and through the evolution of the Internet. In the most familiar sense, a hacker is anyone who attempts to break into computer systems by surreptitious means. But, as you'll see, there's more to being a hacker than simply trying to steal a password or break into someone's Facebook account.
There are hackers who work for large companies whose purpose is to protect the companies for whom they work. There are hackers who hack for the pure pleasure of hacking. There are hacktivists who hack to protest corporate or political policy. There are hackers who get paid to hack accounts, passwords, CAPTCHA and other lucrative targets. And, there are those hackers who just want to exploit, embarrass and expose. They're all hackers.
The word hacker still carries negative connotations because good hackers are never profiled in movies, TV, book and other media forms. To deflect some of this negative perception, hackers attempted to create a related term: cracker. Hackers coined cracker as a reaction to journalists who misrepresented all hackers as criminals. You hardly hear the term cracker anymore related to computer system compromises. It never really caught on as an alternative term for those with unsavory intentions.
Hacker now refers to anyone, regardless of intention or perspective, who attempts to compromise computer systems.
Perhaps the best place to glean a definition of this obscure term is from Eric Raymond's now famous Jargon File:
[originally, someone who makes furniture with an axe]
1. A person who enjoys exploring the details of programmable systems and how to stretch their capabilities, as opposed to most users, who prefer to learn only the minimum necessary. RFC1392, the Internet Users' Glossary, usefully amplifies this as: A person who delights in having an intimate understanding of the internal workings of a system, computers and computer networks in particular.
2. One who programs enthusiastically (even obsessively) or who enjoys programming rather than just theorizing about programming.
5. An expert at a particular program, or one who frequently does work using it or on it; as in ‘a Unix hacker’. (Definitions 1 through 5 are correlated, and people who fit them congregate.)
6. An expert or enthusiast of any kind. One might be an astronomy hacker, for example.
7. One who enjoys the intellectual challenge of creatively overcoming or circumventing limitations.
8. [deprecated] A malicious meddler who tries to discover sensitive information by poking around. Hence password hacker, network hacker. The correct term for this sense is cracker.
The term ‘hacker’ also tends to connote membership in the global community defined by the net (see the network. For discussion of some of the basics of this culture, see the How To Become A Hacker FAQ. It also implies that the person described is seen to subscribe to some version of the hacker ethic (see hacker ethic).
It is better to be described as a hacker by others than to describe oneself that way. Hackers consider themselves something of an elite (a meritocracy based on ability), though one to which new members are gladly welcome. There is thus a certain ego satisfaction to be had in identifying yourself as a hacker (but if you claim to be one and are not, you'll quickly be labeled bogus). See also geek, wannabee.
This term seems to have been first adopted as a badge in the 1960s by the hacker culture surrounding TMRC and the MIT AI Lab. We have a report that it was used in a sense close to this entry's by teenage radio hams and electronics tinkerers in the mid-1950s.
After seeing the definition, do you want to be a hacker?
As with any specialized field, hacking has its own language and culture. To be brought into that culture, you have to prove yourself worthy. It's the same for any IT-related field. Everyone thinks of himself as an expert and everyone else is out to prove him wrong. The hacking subculture is more harsh than mainstream IT in the "prove yourself" category.
If you don't want to risk your freedom to prove yourself, my advice is to study ethical hacking at one of the online sites or to purchase a book on the topic. In an ethical hacking course, you'll learn the tools, techniques, methods and ethics of a true security professional. By learning in a formal atmosphere, you'll gain experience without the risk associated with certain types of penetration (pen) testing.
Remember that the only difference in ethical hacking and unethical/illegal hacking is that person who performs the attacks does so with authorization by the target. Always do so with authorization. There's money to be made doing so and the only time you'll spend behind bars is when you're moonlighting as a bartender working your way through the Ethical Hacker coursework.