The document discusses client-server architectures. It describes the fat server model, which places more functions on the server, and the fat client model, which places more functions on the client but stores data on the server. It then explains the key aspects of two-tier and three-tier client-server architectures. The two-tier architecture involves clients communicating directly with servers, while the three-tier architecture introduces a middle tier between clients and servers to improve performance, scalability, and other advantages.