::''"systemd is a '''system''' and '''session manager''' for Linux, compatible with SysV and LSB init scripts. systemd provides '''aggressive parallelization''' capabilities, uses '''socket''' and '''D-Bus''' activation for starting devices."''

:Upon computer boot-up, and after the Kernel process is started, it traditionally launched the '''init''' process (usually PID 1). This important process manages (launches) other common services. The init process also has the ability to manage process (for example, respawing or "restarting" processes if they are terminated for some reason).

:In many ways, the init process is the "ancestor process" and any process that is currently running on the Unix/Linux system is either directly or indirectly related to the init process.

:Traditionally, the '''init program would run default processes''' that were defined in "shell scripts" contained in appropriate '''run-level''' directory. The run level is a defined state that the Unix/Linux system is currently in (for example, graphical-mode, text-based mode with networking, text-based mode without networking, etc).