What is the difference between "systemctl start" and "systemctl enable"?

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.

disable and 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.

Source: systemctl man page

From the systemctl manpage:

enable NAME...
   Enable one or more unit files or unit file instances, as specified
   on the command line. This will create a number of symlinks as
   encoded in the "[Install]" sections of the unit files. After the
   symlinks have been created, the systemd configuration is reloaded
   (in a way that is equivalent to daemon-reload) to ensure the
   changes are taken into account immediately. Note that this does not
   have the effect of also starting any of the units being enabled. If
   this is desired, either --now should be used together with this
   command, or an additional start command must be invoked for the
   unit.
   ...
   Enabling units should not be confused with starting (activating)
   units, as done by the start command. Enabling and starting units is
   orthogonal: units may be enabled without being started and started
   without being enabled. Enabling simply hooks the unit into various
   suggested places (for example, so that the unit is automatically
   started on boot or when a particular kind of hardware is plugged
   in). Starting actually spawns the daemon process (in case of
   service units), or binds the socket (in case of socket units), and
   so on.

Essentially, enable marks the service for starting up on boot, and start actually starts the service immediately.

As of systemctl version 220, enable and disable support a --now switch to start / stop services concurrent with the enabling / disabling.

e.g. systemctl --now enable foobar.service

Use systemctl --version to check your installed version.