It is the process of evaluating the security of an organization by exploiting the vulnerabilities in a way the attackers could exploit them and thereby defending as well as documenting the procedure of attack.
Types of penetration testing:
Black box: The penetration tester will not be given any details pertaining to the network, or infrastructure of the network/ organization
White Box: the penetration tester will be aware of the complete details of the infrastructure to be tested
grey box: The penetration tester will be provided with a limited knowledge about the systems to be tested.