When computers were first linked together into networks, moving information between the different types of computers was a very difficult task. In the early 1980s, the International Standards Organization (ISO) recognized the need for a standard network model. This would help vendors to create interpretable network devices. The Open Systems Interconnection (OSI) reference model, released in 1984, addressed this need. The OSI model describes how information makes its way from application programs through a network medium to another application program in another computer. It divides this one big problem into seven smaller problems. Each of these seven problems is reasonably self-contained and therefore more easily solved without excessive reliance on external information. Each problem is addressed by one of the seven layers of the OSI model. The seven layers of the OSI model are: -

Application - It provides services to application programs outside the scope of the OSI model.
Presentation - It ensures that information sent by one system will be readable by another system.
Session - The main function of the OSI model's session layer is to control "sessions".
Transport - It provides a data transport service that shields the upper layers from transport implementation issues.
Network - The network layer sends packets from source network to destination network.
Data-link - It is responsible for providing reliable transit of data across a physical link.
Physical - It is concerned with the interface to the transmission medium.

The acronym used to remember these layers is: All People Seem To Need Data Processing.
The lower two OSI model layers are implemented with hardware and software.
The upper five are generally implemented only in software.