OSI Reference Model

Application Layer:

Application Layer acts as a interface between the application and the next layer. Application Layer does not mean the applications like IE.But the protocols like telnet,FTP, email client(SMTP), Hyper Text Transfer Protocol (HTTP) which provides services to the application.

Presentation Layer

This layer presents data in a particular format to the application layer.Examples of data formats like ASCII text, images are presented in the format of JPG, GIF or PNG. The functions like data compression,decompression, encyption and decryption come under this layer.

Session Layer

This layer provides communications between two devices by creating, managing and ending the sessions between the applications of both devices. The communication is done via three modes called simplex,half-duplex and full duplex.
Simplex means one way communication. The sender just sends the data and it will not receive any reply.
Half-duplex is a two way communication but at any point of time the communication will happen in only one direction.
Full-duplex is a two way communication and communication can happen in both directions at any given time.

Transport Layer

This layer is responsible for safe transport of data between the two hosts. This layer collects the data from the upper layers,breaks and reassembles the data in the even way to transport to the destination host. Transmission control Protocol(TCP) and User Datagram Protocol(UDP) are part of this layer.TCP is reliable but UDP is not reliable.

Network Layer

The job of this layer is to provide addressing of the devices so that better communication is possible.Routers come in this layer. Routers help in routing the data from source to destination.

Data Link Layer

This layer consists of protocols for a particular type of Physical medium.The data from the Network layer is converted in to bits and are transmitted over the physical layer. This layer formats the messages into a data frame and adds the hardware source address and the desination address.

Physical Layer

This layer defines the physical characteristics of the transmission medium like the voltage, connectors, timings etc.