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.
*specify how a pair of applications interact when they communicate *specify details about  - the format  - the meaning of messages that applications can exchange
layer 5 in internet model, the software that enables the users to perform useful work  ex) instant messaging, videoconference, file transfer, email, web, wordprocessing