provides an interface from the application to the network by supplying a protocol with actions meaningful to the application, for example, &"get web page object.&"
layer 5 in internet model, the software that enables the users to perform useful work  ex) instant messaging, videoconference, file transfer, email, web, wordprocessing
where the application using the network resides, common network applications include web browsing, e-mail, file transfers, and remote logins.