Basic Terms

Certified Ethical Hacker or Penetration Tester is someone who uses same hacking techniques to test the security of an organization that an actual hacker would use to be able to compromise it. The difference basically comes down to motive.

       One of the definition of a hacker is, somebody who enjoys learning the details of computer systems and how to stretch out their own capability; as opposed to just generic users, who only want to learn the minimal amount necessary.

Hacker  -  A hacker is a person who modifies something to perform in a way that differs from its original design or intent. That doesn’t always refer just to computer issues, but that’s typically how we think of it today.

Cracker -  A cracker is defiantly someone who is typically viewed as someone who hacks into a computer for offensive purposes. A cracker is still a kind of hacker.

Ethical Hacker - Ethical Hacker is a security professional who uses hacking skills for defensive purposes.

White Hat - An ethical hacker should always be a white hat hacker. This is a hacker with defensive security intentions. In the past ethical hackers were referred to as white hat hackers.

Black Hat - Black Hat Hacker is a hacker with malicious or destructive intentions, somewhat synonym with a cracker.

Grey Hat – It is a combination between a white hat and a black hat hacker. Typically a grey hat hacker is a person who alternates roles as a white hat or black hat hacker. I really think most white hat or ethical hackers are really grey hat hackers.

Script Kiddy - This is somebody who uses hacker’s tools indiscriminately or unskillfully, and they never come up with anything original. They just take a bunch of tools and keep launching them against a target, because they don’t really know what else to do.
                          Regarded as a highly insulting term.

Hacktivism - For example: A Hacktivist will typically deface a website to make their point.

Vulnerability – It is simply a weakness that could lead to compromised security.

Exploit - An exploit is a defined method of hacking a vulnerability, or it’s a proof of concept.

0Day - An unreported exploit. The person who comes up with the exploit first has come up with a 0Day. They are the originator of an exploit against a vulnerability.

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           Penetrate only what you have permission to penetrate.