The Internet protocol suite is comprised of protocols that generate the software systems of the Internet. The application layer of the Internet protocol suite is viewed as the highest layer of the Internet protocol suite. The Internet protocol suite has four layers, the lower the layer, the closer the layer is to the physical transmission of data.
The protocols of the application layer typically provide the user services of the Internet. Some of the most popular protocols of the application layer are as follows:
Data generated by the application layer protocols is (typically) encapsulated by transport layer protocols. The TCP and UDP protocols of the transport layer convert application layer data into TCP and UDP data units. Through encapsulation, TCP and UDP data unts will be converted into IP data units (Internet layer) and finally frame data units (Link layer). When the data has been passed through the four layers, data will have been transported from it's source to it's destination.
Application layer protocols view transport layer protocols - like TCP and UDP - as providing a service which allows them to communicate across the Internet. Application layer protocols typically have no knowledge of the internal workings of transport layer protocols. Application layer protocols are typically associated with a client-server application: web browser, email client, or download manager. These client applications are unconcerned about how transport layer protocols communicate their data across networks, they are only concerned that the data is transported in an accurate manner.