The programming model used in distributed computing systems is typically the client-server model. In this architecture, clients request resources or services from centralized servers that manage databases and application logic. This separation of concerns optimizes resource utilization and allows for scalable architectures. Another common model is peer-to-peer (P2P), where nodes operate collaboratively without a dominant server. Both models facilitate the processing and distribution of data across multiple systems, enhancing performance and reliability in distributed environments while allowing for efficient handling of large-scale computational tasks.