What is the difference between service and systemctl?

Maybe this is a trivial question, but it is not totally clear to me. On one of our servers we have some background processes running which were started with service and some others which were started with systemctl, like this:

$ service nginx start
$ systemctl start gunicorn

What is the difference between the two commands? Which one is the preferred way to deal with background services? How to configure the preferred command?

service is an "high-level" command used for starting and stopping services in different unixes and linuxes. Depending on the "lower-level" service manager, service redirects on different binaries.

For example, on CentOS 7 it redirects to systemctl, while on CentOS 6 it directly calls the relative /etc/init.d script. On the other hand, in older Ubuntu releases it redirects to upstart

service is adequate for basic service management, while directly calling systemctl give greater control options.

systemctl is basically a more powerful version of service.

With service you can only do commands related to the service (i.e. status, reload, restart) whereas with systemctl you can use more advanced commands such as:

systemctl is-failed name.service # check if service failed to load

Or masking services:

systemctl mask name.service

There's a lot of good info on this page from Ask Ubuntu.