An OSI layer is a seven layer model in which layers provide services to its upper layers and take services from lower layers. Each layer interacts with its peer layer via a protocol.It transmits PDU called as protocol data unit.The seven layers are given below:

Layer7: Application layer

In this layer, communication partners and quality of service are identified and user authentication and privacy are considered. This layer supports application and end-user processes.The layer provides application services for file transfers,e-mail and other network software services.

Layer6: Presentation layer

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer.

Layer5: Session layer

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination.

Layer4: Transport layer

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer.

Layer3: Network layer

This layer provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing.

Layer2:Data link layer

At this layer, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.

Layer1:Physical layer

This layer conveys the bit stream - electrical impulse, light or radio signal -- through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects.

Some important things to remember:

Hubs/Repeaters are found in the Physical Layer

Switches /Bridges/ are found in the Data Link Layer

Routers are found in the Network Layer

Gateway are found in All 7 of the OSI Layers