The document discusses client-server architecture. It describes the basic components of client-server systems including clients that make requests, servers that process requests and return responses, and a communication network that connects them. It outlines two main types of client-server architectures: 2-tier with a direct client-server connection, and 3-tier with middleware between the client and server. The document also distinguishes between thin and thick clients, iterative and concurrent servers, and three common client-server models.