The main function of application layer software is
responsible for providing an interface to allow programs to
use network services. The term "application layer" does not
mean running on the network of a particular application, the
application layer provides services including file transfer,
file management and e-mail the information processing.
Communicate with other computers an application; it is the
corresponding application's communication services. For
example, word processors without communication function
cannot execute the code to communicate, engage in
word-processing work of programmers do not care about
OSI Layer 7. However, if you add a file transfer option, then
the word processor programmer's need to implement OSI
Layer 7.
Application layer protocols include: Telnet, FTP, HTTP,
SNMP and so on.