Communication on the Internet network is governed by various protocols. These protocols, or rules, spell out how the participants in various network processes should behave. Application protocol is one such protocol.
Application protocols govern various processes, such as the process for downloading a web page, or for sending e-mail. The application protocol directs how these processes are done.
An application protocol is layered on top of a transport protocol. The application protocol provides the bytes that carry the messages and the responses for various processes, and the transport protocol helps reliably transport the bytes and makes sure they arrive in order.
In addition to the application and transport protocols, there are also other layers involved in enabling network communication, such as network protocol. All of them come together to facilitate the process whereby various applications communicate with each other.