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The Internet of Things (IoT) has emerged as a transformative technology by enabling billions of interconnected devices to communicate, exchange data, and provide intelligent services across diverse application domains such as healthcare, smart cities, agriculture, industrial automation, and transportation. Despite its widespread adoption, the heterogeneous nature of IoT devices, resource constraints, and the increasing sophistication of cyber-attacks have introduced significant security and privacy challenges. Ensuring the security of IoT environments has therefore become a critical requirement for protecting sensitive data, maintaining service availability, and preserving user privacy. This paper presents a comprehensive review of IoT security by examining recent research trends, fundamental security requirements, major threat environments, and practical security guidelines. The study discusses essential security requirements, including confidentiality, integrity, availability, authentication, authorization, non-repudiation, and data freshness. Furthermore, it analyzes security threats at the device, network, cloud, and application layers and summarizes practical measures for developing secure IoT systems. The paper also highlights recent advancements in lightweight authentication, zero-trust security, artificial intelligence-assisted threat detection, and privacy-preserving techniques that strengthen modern IoT ecosystems. The review provides a concise yet comprehensive overview of IoT security concepts and serves as a useful reference for researchers, practitioners, and students interested in developing secure and reliable IoT applications.