os:process:service

About

A service is a process that:

It's declared and managed by the OS init system. With the init system, you may:

It is used remotely through some remote interface, either synchronous or asynchronous (eg web service, messaging system, RPC, or socket.)

To have a list of the well-known services, you can inspect the services file (The services translation file) on linux, located at:

/etc/services

Example which port used the port 25?

cat /etc/services | grep ' 25/tcp'

This is a SMTP port

smtp            25/tcp          mai

A service is identified:

on the network by a socket. The most well-known is the netsocket (ie the hostname, the port and the protocol)
on the application level by its scheme (application protocol)

For instance, if you known that there is a service on the port 80, you would expect it to be: