OSI - Open Systems Interconnection
The Open Systems Interconnection model is a layered system architecture that defines 'services' in different layers of the vertical stack. This layered architecture was designed by the ISO (International Standards Organization) in the year 1983.
'Open Systems Interconnection' is just a framework, and not a "physical" entity. It does not perform any task in the networking process. It is just used to understand the complex interaction between the networking devices and information flow.
The OSI model divides each and every function of a network into a separate layer. So, if you are working in layer 3, you don't need to worry about devices and protocols that resides at other layers.
The OSI model comprises of 7 layers namely:-
Layer 1 - Physical layer
Layer 2 - Data Link Layer
Layer 3 - Network Layer
Layer 4 - Transport Layer
Layer 5 - Session Layer
Layer 6 - Presentation Layer
Layer 7 - Application layer