The document discusses the Open Systems Interconnection (OSI) model, which defines seven layers of network communication. The seven layers are the physical, data link, network, transport, session, presentation, and application layers. Each layer has a specific function, with the lower layers dealing with physical connectivity and data transmission and the higher layers focusing on software applications and end-user interactions. The document provides details on the functions of each layer, such as the physical layer defining connections, signals, and topologies, the data link layer handling framing, addressing, and error control, and the transport layer providing service addressing, segmentation/reassembly, and connection control.