The document discusses the Presentation-Abstraction-Control (PAC) pattern for developing web applications. PAC defines a hierarchical structure of cooperating agents, where each agent is responsible for a specific aspect of the application. Each agent consists of three components: presentation, abstraction, and control. The presentation component handles human-computer interaction, the abstraction component manages the data model, and the control component mediates communication between agents. The PAC pattern supports modularity, change, extension, and multitasking through separation of these components within agents.