This research paper explores common security threats in client-server communication, which is the foundation of most modern digital systems. It begins by explaining the structure and types of client-server architecture, including two-tier and three-tier systems, before examining key cybersecurity threats such as Man-in-the-Middle (MitM) attacks, Session Hijacking, Cross-Site Scripting (XSS), Cross-Site Request Forgery (CSRF), and SQL Injection. Each attack is explained in simple and clear language, including how it occurs and its impact on client-server communication. The paper also provides detailed preventive measures and best practices, emphasizing encryption, input validation, authentication, and user awareness. The goal of this study is to help readers, especially students and beginners in cybersecurity, understand how these threats work and how to protect client-server systems from them.