This research paper presents a comprehensive study of cloud computing, focusing on its architectures, service and deployment models, security challenges, and real-world applications. The paper explores core cloud service models—Infrastructure as a Service (IaaS), Platform as a Service (PaaS), and Software as a Service (SaaS)—along with deployment strategies such as public, private, hybrid, and community clouds. It examines enabling technologies including virtualization, containerization, and container orchestration (Docker and Kubernetes), highlighting their impact on scalability, performance, and isolation. A significant emphasis is placed on cloud security, covering threats such as data breaches, misconfigurations, insecure APIs, identity and access management issues, and multi-tenancy risks. Modern security approaches like Zero Trust Architecture, DevSecOps, cloud security automation, API security, and container security are discussed in detail. To demonstrate practical implementation, the paper proposes a serverless cloud-based automated receipt/document processing architecture using AWS services such as S3, Lambda, Textract, DynamoDB, SES, and CloudWatch. The architecture is evaluated in terms of performance, cost, scalability, and security. Finally, the paper identifies emerging research directions including multi-cloud orchestration, quantum-resistant cryptography, privacy-preserving computation, sustainable cloud computing, and automated security management, emphasizing the need for resilient and secure cloud systems in future computing environments.