Penetration testing, or 'pen testing', is the process of evaluating systems to identify vulnerabilities from a non-privileged user perspective, aimed at enhancing security and compliance. It involves various stakeholders like internal employees, security consultants, and third-party auditors, and combines both technical and non-technical skills. The document outlines common approaches, required skills, tools used, and career paths in penetration testing while emphasizing the importance of responsible hacking.