Open System Interconnection (OSI)

OSI is a seven-layer network model specified by ISO. It is formally referred to as the Basic Reference Model for Open Systems Interconnection. The seven layers are:

Layer 7 - ApplicationOSI Layers
This layer is concerned with the application related to the communication between systems. This layer would convey quality of service parameters, authenticate users, and verify that the protocol exchanges are correct (e.g., ensure that a message is not transmitted that would result in a protocol violation).
Layer 6 - Presentation
This is the layer that is responsible for digesting the received messages and "understanding" them or properly composing messages that may be understood by the other device. This might include the formation of a stream of data formatted according to the HTTP protocol specifications or encoding a message according to an ASN.1 protocol specification.
Layer 5 - Session
This layer is responsible for setting up and managing sessions or dialogs between two applications or services. It relies on the transport protocol underneath to deliver messages. In the world of IP, this might be viewed as the TCP APIs or routines that tranmit and receive UDP packets in an application.
Layer 4 - Transport
This is the layer at which differentiation is made between applications or services on the network from the underlying protocols. In the world of IP, this equates to TCP and UDP, though TCP also falls a little into layer 5, as well.
Layer 3 - Network
The network layer defines the protocol that allows for communication between various system and applications on any number of network links. The Internet Protocol is an example of a protocol that operates at Layer 3.
Layer 2 - Datalink
The datalink layer is the lowest layer at which some defined message is transmitted. This layer includes protocols like Ethernet. Messages at this layer contain sufficient information about the destination to be properly delivered to the proper network interface on a host on a single single link.
Layer 1 - Physical
This layer refers to the physical medium (e.g., fiber optic cable, twisted-pair cable, coaxial cable, etc.) through which a device is connected.

Resources: Understanding OSI