What is Package & Package Manager in Linux?
A package is usually referred to an application but it could be a GUI application, command line tool or a software library (required by other software programs). A package is essentially an archive file containing the binary executable, configuration file and sometimes information about the dependencies.
A package manager is a tool that allows users to install, remove, upgrade, configure and manage software packages on an operating system. The package manager can be a graphical application like a software center or a command line tool like apt-get or pacman.
"systemctl" and "systemd":
"systemctl" is used to examine and control the state of “systemd” system and service manager. "systemd" is system and service manager for Unix like operating systems(most of the distributions, not all).
"systemctl" vs "service":
The "systemctl" command manages both system and service configurations, enabling administrators to manage the OS and control the status of services. Further, systemctl is useful for troubleshooting and basic performance tuning.
The "service" command starts, stop and restart a daemon or services by calling the script. Usually all scripts are stored in /etc/init. d directory. It runs a script in as predictable environment as possible.