The Application Layer is the seventh layer of the OSI model which provides services for user applications and also supports the network access. Although the transport layer handles most of the interfacing between the network and applications, the Application Layer in the OSI model is still needed. It focuses more on the network services, API’s, utilities, and operating system environments. Despite this, there are three ODI model layers that correspond to the TCP/IP Application Layers. The breaking down of the application layer into three layers helps to simplify the actual responsibilities of the layer for better understanding. http://learn-networking.com/tcp-ip/how-the-application-layer-works
The application layer is the layer that is actually used by the network applications. It implements functions performed by users so the user’s tasks can be accomplished over the network. It is important not to let the name of this layer confuse you, as it is not exactly what you would think of when you hear the word application. This layer actually uses the services offered by a protocol that operates in the Application Layer. Not all user applications actually use the Application Layer of the network the same ways. For example, if a text editor is used to open a file on another computer on your network, the application layer will not be used; it doesn’t even know the file is even on the network. All it sees is a file addressed with a name that has been mapped to another network. The operating system actually takes care of the redirecting components over the network. Not all uses of the application layer are actually used by applications. The operating system itself can and will use services directly at the application layer. There are numerous application layer protocols which enables numerous functins at the Application Layer. Some examples of these are: HTTP, FTP, DHCP, Telnet, and IRC. This is the only layer that does not provide any services to the layer above it being that it is the top layer. It provides the actual services to the network and the users. Its main responsibilities so to speak are to implement functions needed by users of the network and issue commands for lower layers to use. http://www.tcpipguide.com/free/t_ApplicationLayerLayer7.htm
The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the presentation layer below it. http://www.inetdaemon.com/tutorials/basic_concepts/network_models/osi_model/application.shtml