The application layer is the seventh level of the seven-layer OSI model, and the fifth layer of the five-layer TCP/IP model. It interfaces directly to and performs common application services for the application processes; it also issues requests to the presentation layer.
The common application layer services provide semantic conversion between associated application processes. Note: Examples of common application services of general interest include the virtual file, virtual terminal, and job transfer and manipulation protocols.
The application layer of the four-layer and five-layer TCP/IP models corresponds to the application layer, the presentation layer and session layer in the seven layer OSI model.
Reference:
http://en.wikipedia.org/wiki/Application_layer
