systemctl start and
systemctl enable do different things.
enable will hook the specified unit into relevant places, so that it will automatically start on boot, or when relevant hardware is plugged in, or other situations depending on what's specified in the unit file.
start starts the unit right now.
stop are the opposite of these, respectively.
This means that when you first install MariaDB, you might want to run
systemctl enable mariadb.service to enable it so it starts on boot. You might also want to run
systemctl start mariadb.service, or just reboot, in order to start MariaDB. To stop MariaDB, run
systemctl stop mariadb.service (it will start again on next boot or when you manually start it). To disable it so it doesn't start on boot anymore, run
systemctl disable mariadb.service.
As noted in gerardw's answer, starting from version 220, released in may 2015, both
disable started to take the optional
--now switch in order to also start or stop the unit, depending on the used command.
To both disable and stop a unit with the same command, use
systemctl disable mariadb.service --now. Similarly, to both enable and start a unit, use
systemctl enable mariadb.service --now.
Source: systemctl man page