This document discusses application layer protocols and services in computer networking. It begins by describing the seven-layer OSI model and how the application layer fits within this model. It then explains how several important TCP/IP application layer protocols function, such as HTTP, DNS, SMTP, and FTP. The document also discusses the client-server and peer-to-peer networking models and how application layer protocols enable communication between clients and servers or peers. It concludes by defining port numbers used by common application layer protocols and services.