A network service is a program running on a local machine that other machines can connect to and use over a network. Common examples include web, email, and file-transfer servers.
This chapter describes how network services work in general, and how several of the more popular services work on Mac OS X.
Generally, a network service operates through a daemon program that listens for incoming connections on a certain port; web servers usually listen on port 80, for example, and ssh connections typically happen on port 22. (The precise way it accomplishes this is implementation-specific; it might choose to handle the whole connection itself or fork off another process to handle it so the daemon can get back to listening.)