Ethical Hacking is also often termed as Intrusion, Penetration Testing, Tiger Teaming, Sneaking, or Red Teaming. The methodologies used for ethical hacking are similar to those used for malicious hacking. The only difference between the two is the former uses the methods to report problem, the latter uses them to reap profits illegitimately.
Penetration refers to a successful, but an unauthorized breach of security perimeter of a computer system. Penetration Testing is done as a part of ethical hacking, wherein the computer network’s security systems are evaluated thoroughly by simulating attacks from malevolent sources, also called the black hat hackers.
Vulnerability, with regards to computer security, refers to weakness in the computer system or network that enables an attacker to hack or crack into the system and affect its information assurance.