OSI reference model:

It is the model to under stand network.It is layered application to understand how your data moves from one machine to another.
Osi model overview:
7:Application(Reduces complexity)
6:Presentation(Standardizes interfaces)
5:Session(Facilitates modular engineering)
4:Transport(transport of data)
3:Network(include routers and switches)
2:datalink (switches)
1:Physical(end devices)