Cloud storage services have become a fundamental component of modern digital infrastructure, enabling individuals and organizations to store, manage, and access data over the internet. With the rapid growth of digital transformation, remote work, and data-driven applications, the reliance on cloud platforms for data storage has increased significantly. However, this widespread adoption raises serious concerns regarding data security, privacy, and regulatory compliance. This study presents a comparative analysis of widely used cloud storage services including Google Drive, Dropbox, Microsoft OneDrive, and Amazon S3 with respect to their security architectures, encryption mechanisms, authentication models, access control policies, and compliance standards. The paper highlights the strengths and limitations of each service, emphasizing how differences in security design impact data confidentiality, integrity, and availability. By evaluating technical features such as encryption at rest and in transit, identity and access management, audit logging, data redundancy, and compliance with international security standards, this study provides a structured framework to help users and organizations select suitable cloud storage platforms based on security requirements. The findings underline the importance of combining provider-level security controls with organizational policies and user awareness to ensure comprehensive data protection in cloud environments.