OSI

The Open Systems Interconnection project at ISO is a reference model (unlike TCP/IP which is a protocol model).

Layers

7 - Application

The application layer contains the protocols for process to process communication.

6 - Presentation

Provides common representations of the data transferred between layer 7 services.

5 - Session

Like layer 6 does to 7, it provides services to the presentation layer.

4 - Transport

It defines the services for transport data between end devices.

3 - Network

Provides services to exchange data over network between identified end devices.

Provides methods for exchanging data frames over a shared media.

1 - Physical

Describes the hardware, and means of physical connections used for transmission of data.