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