layer 5 in internet model, the software that enables the users to perform useful work  ex) instant messaging, videoconference, file transfer, email, web, wordprocessing
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.&"
first layer in the tcp/ip stack. where networking applications, like web browsers operate. applications select the correct protocol depending on what the user wants to do.