The expansion of digitized business processes, cloud-native infrastructures, and hyperconnected devices has unlocked tremendous value but also widened the attack surface at an unprecedented pace. Signature- and rule-based defenses alone struggle to keep up with polymorphic malware, living-off-the-land techniques, and fast-evolving social engineering campaigns. Artificial Intelligence (AI) offers a data-driven complement: learning from patterns across endpoints, networks, and identities to surface weak signals, prioritize risk, and automate time-critical responses. This paper presents a comprehensive, practitioner-oriented view of AI in cybersecurity. We synthesize the state of techniques—supervised and unsupervised learning, deep representation learning, graph learning for relationships, natural language processing (NLP) for threat intel and phishing, and reinforcement learning (RL) for adaptive defense. We review applications across malware classification, intrusion detection, fraud and account takeover (ATO), email and web security, identity and access management, and security operations (SecOps) automation. We formalize evaluation metrics and datasets, discuss system architecture patterns that make AI operationally useful, and examine limitations including adversarial machine learning, data quality and drift, privacy and governance, model transparency, and the talent gap. We conclude with a forward-looking agenda that emphasizes explainable and trustworthy AI, federated and privacy-preserving learning, robust training against adversaries, and human-in-the-loop collaboration to build proactive, resilient defense capabilities.