The 7 Layers of the OSI Model

The OSI, or Open System Interconnection, model defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.

Application (Layer 7)	This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and any constraints on data  completed. Everything at this layer relies on layers 1 to 6 working efficiently.
Presentation (Layer 6)	The presentation layer works to transform data into the form that the application layer can accept. This layer formats data sent across a network, providing freedom from compatibility problems. Everything at this layer relies on layers 1 to 5 working efficiently.
Session (Layer 5)	This layer establishes, manages and terminates connections ie data conversations / exchanges between applications. Everything at this layer relies on layers 1 to 4 working efficiently.
Transport (Layer 4)	This layer is where the TCP/IP protocol and other communications level protocols exist.  Without TCP, the application will fail, without layers 1 to 3, TCP will fail.   Data validation is now performed - When data gets corrupted at layer 1, TCP will request retransmission and simply wait until a clear packet is received - applications further up the OSI stack are simply put on hold until this is complete.  Everything at this layer relies on layers 1 to 3 working efficiently.
Network (Layer 3)	This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.  Everything at this layer relies on layers 1 and 2 working efficiently.
Data Link (Layer 2)	At this layer, data packets are encoded and decoded into bits. In practice, layer 2 is the network interface hardware - ie the NIC card.  Everything at this layer relies on layer 1 below working efficiently.
Physical (Layer 1)	This ls the layer we install. Everthing else depends on our workmanship, which conveys the data stream. It provides the hardware and the means of sending and receiving data by the carrier protocols . Ethernet, Fast Ethernet, RS232, and ATM are protocols with physical layer components.

This graphic is taken from The Abdus Salam International Centre for Theoretical Physics.