A service in nagios defines a particular test which should be run. At a minimum you need to define a name for the service and the command to run in order to monitor it.
Similar to hosts, it is defined within .cfg files in /etc/nagios3/conf.d or a subdirectory. At a technical level, there is no difference between a .cfg file that defines a host versus one that defines a service. They are split in Ubuntu's default configuration just for ease of management. If you wanted to, you could have a single flat .cfg that defines all hosts, services, and users.
Again I like to split my services into a subdirectory, so let's look at defining a service to monitor HP Jetdirect printers by creating /etc/nagios3/conf.d/services/printer.cfg containing:
define hostgroup {
hostgroup_name printers
}
define service {
hostgroup_name printers
service_description jetdirect
check_command check_hpjd
...
