The seven-layer OSI Networking model

Application layer (layer 7): The topmost layer of the OSI model provides a means for users to actually access network resources.  This is the only layer typically seen by end users, as it provides the interface that is the base for all of their network activities. 

Presentation layer (layer 6): This layer transforms the data it receives into a format that can be read by the application layer.  The data encoding and decoding done here depends on the application layer protocol that is sending or receiving the data.  The presentation layer also handles several forms of encryption and decryption used for securing data.  The presentation layer provides a variety of coding and conversion functions that are applied to application layer data.  These functions ensure that data sent from the application layer of one system can be read by the application layer of another system.  An example of coding functions is the encryption of data after it leaves an application.  Another example is the JPEG and GIF formats of images displayed on web pages.  This formatting ensures that all web browsers, regardless of operating system, can display the images.

Session layer (layer 5): This layer manages the dialogue, or session between two computers.  It establishes, manages, and terminates this connection among all communicating devices.  The session layer is also responsible for establishing whether a connection is duplex or half-duplex, and for gracefully closing a connection between hosts, rather than dropping it abruptly.

Transport layer (layer 4): The primary purpose of the transport layer is to provide reliable data transport services to lower layers.  Through flow control, segmentation/desegmentation, and error control, the transport layer makes sure data gets from point to point error-free.  Because ensuring reliable data transportation can be extremely cumbersome, the OSI model devotes an entire layer to it.  The transport layer utilizes both connection-oriented and connectionless protocols.  Certain firewalls and proxy servers operate at this layer. 

Network layer (layer 3): This layer is responsible for routing data between physical networks, and it is one of the most complex of the OSI layers.  It is responsible for the logical addressing of network hosts (for example, through an IP address).  It also handles packet fragmentation, and in some cases, error detection.  Routers operate at this layer.

Data link layer (layer 2): This layer provides a means of transporting data across a physical network.  Its primary purpose is to provide an addressing scheme that can be used to identify physical devices (for example, MAC addresses).  Switches operate at the data link layer.

Physical layer (layer 1): The layer at the bottom of the OSI model is the physical medium through which network data is transferred.  This layer defines the physical and electrical nature of all hardware used, including voltages, hubs, network adapters, repeaters, and cabling specifications.  The physical layer establishes and terminates connections, provides a means of sharing communication resources, and converts signals from digital to analog and vice versa.