OSI layers (Open Systems Interconnection) Reference model connect systems that are open for communication with other systems.

It is a reference tool to understanding data communications between any two networked systems. It divides the communications into seven layers. Each layer performs specific functions to support the layers above it and the layers below it.The function of each layer is defined according to the internationally standardized protocols.This model allows all network elements to operate together, no matter who created the protocols and what computer vendor supports them.

Why do we need OSI model ?

Computer / Network is a complex architecture and its functions are divided into different group of hardware. When networking started, Computer can communicate with computers of same manufacturers. The purpose of the OSI model was to assist vendors to produce inter-operable network systems.

To establish communication between computers / Networks of different vendors

Benefits of OSI layers

• Communication between different vendors.
• How hardware and software elements function together.
• Troubleshooting easier by separating networks into manageable pieces.
• It helps to understand new technologies as they are developed.

Layering and Headers

Each layer needs to add some control information to the data in order to complete it’s job. This information is typically pre-appended / separated to the data before being given to the Next layer. Once the lower layers deliver the data on other side, the peer layer uses the control information to handle that data. All upper 06 layers insert header information only except data link layer which insert trailer information also. Physical layer do not add any header or trailer information.

 Application Layer

It provides services to the user application to communicate over the internet.enables user or software to get access to the network

• Internet browsing using HTTP
• File transfer in remote computer using FTP
• Email services using SMTP
• etc

Presentation Layer

It ensure that data transfer from the application layer of one system is readable by the application layer of the second system

• Syntax and semantics = Language selection
• Data translation (Alphabet, numbers into bits)
• Compression and decompression
• Encryption and decryption

Session Layer

• synchronize the conversation between two different applications.
• Establish Connection
• Security and access control
• Dialogue control. Duplexing (half, full)
• End connection

Transport Layer

• Delivery of entire message from source to destination.
• Port addressing
• TCP and UDP(reliable and unreliable communication)
• Segmentation and reassembly
• Flow control
• Error control
• Segment

Network Layer

• Source to destination delivery of packet (end to end)
• Logical addressing ( IP address )
• Routing, Router operates in network layer. 
• packet

Data Link Layer

• Divide message into pieces called frame
• Node to node delivery of data
• Error control,flow control
• Physical addressing (MAC = Medium access control)
• Framing

Physical Layer

• Directly deals with physical media.
• Representation of bits
• Encoding Changes bits into voltage levels
• Topology.
• Data rate.

All these layers are logically separated in a system.You can not physically find any distribution in any system.