What is OSI reference model (Open Systems Interconnection)?

Transmission media stays outside the scope of the Physical Layer and are also referred to as Layer 0 of the Open Systems Interconnection (OSI) Model.

how applications can communicate with each other over a network

it is conceptual model it has7 layers from bottom to up

7.  Application layer

6.  Presentation layer

5.  Session layer

4.  Transport layer

3.  Network layer

2.  Data link layer

1.  Physical layer

The seven layers use to establish the connection through3-way handshaking between client program and server program are called7-layers O.S.I model. It includes application, presentation, session, transport, network, data-link and physical(only touchable layer).