This chapter provides a comprehensive exploration of cybersecurity fundamentals, examining the evolving threat landscape and the critical role of ethical hacking in modern information security. We begin with foundational concepts, including the CIA triad and established frameworks, then progress through contemporary threats, vulnerabilities, and defence mechanisms. The chapter emphasises practical applications through case studies of significant security incidents and successful implementations, concluding with emerging paradigms such as zero-trust architectures and quantum-safe cryptography. This treatment serves as both an introduction to cybersecurity principles and a foundation for understanding the intersection of computer science and information security.