Open Systems Interconnect. The OSI model for communications protocols is a global ISO standard for communications that contains protocols in seven layers. Control is passed from one layer to the next, starting at one end, proceeding through the layers to the other and back again. The following chart names the layers and their functions:

Layer 1 - Physical Layer - The transmission medium itself (twisted pair, fiber optic, free-space optics, etc.)
Layer 2 - Data-link Layer - Receives and transmits data over the physical layer.
Layer 3 - Network Layer - Switching and routing layer.
Layer 4 - Transport Layer - Responsible for error correction and direction of flow (transmit/receive).
Layer 5 - Session Layer - Establishes and maintains connection to the communications processes in the lower layers.
Layer 6 - Presentation Layer - Encodes and decodes the data to be transmitted.
Layer 7 - Application Layer - Connects an application or program to a communications protocol.