This report provides a comprehensive analysis of Distributed Denial of Service (DDoS) attacks, detailing their operational mechanisms and real-world impact, with a specific focus on the Operation Ababil incident. The report begins with an overview of computer security and the increasing need for it due to rising cybercrimes. It explains how DDoS attacks work, including the roles of master computers, victim servers, and botnets, and how attackers exploit vulnerabilities to flood networks with traffic, rendering services unavailable to legitimate users. The report then delves into Operation Ababil, a series of cyber-attacks targeting American banks in 2012 and 2013, discussing its motivations, impact on financial institutions, and the breaches in security principles, particularly availability. Finally, it touches on mitigation strategies, such as monitoring internet traffic, using firewalls and anti-viruses, implementing disaster recovery plans, and encrypting sensitive data, to safeguard against such attacks.