start (activate) a service , you will run the command
systemctl start my_service.service, this will start the service immediately in the current session.
enable a service at boot , you will run
systemctl enable my_service.service .
Enable one or more units or unit instances. This will create a set of symlinks, as encoded in the "[Install]" sections of the indicated unit files. After the symlinks have been created, the system manager configuration is reloaded (in a way equivalent to daemon-reload), in order to ensure the changes are taken into account immediately
init scripts , when you type
systemctl enable to start a service at boot it will be linked to
#systemctl enable my_service.service
ln -s '/usr/lib/systemd/system/my_service.service' '/etc/systemd/system/multi-user.target.wants/my_service.service'