With systemctl you operate systemd. To understand systemd, you need to be familiar with this terminology:
Unit file — Encodes information about things such as a service, socket, device, mount, automount, target, snapshot, etc.
Target — A unit configuration file whose name ends in “.target” encodes information about a target unit of systemd, which is used for grouping units and as well-known synchronization point during startup.
Slice — A concept for hierarchically managing resources of a group of processes.
Seat — The set of hardware available at one work place (graphics card, keyboard, mouse, usb devices).
Session — A session is created once a user is logged on,using a specific seat:
– Only one session can be active per seat.
– The default seat (for Linux consoles) is seat0. Hardware is assigned to seats, such as a VGA graphics card