systemd is a new replacement to the System V init system long used in all flavor of UNIX and Linux systems. Ok! ok! not all flavor of Linux/Unix.. Solaris switched a long time ago to use SMF while Ubuntu/Fedora (and the distribution targeted to end users) use Upstart - and yes if you consider Darwin(OSX) a true Unix system :-X, then those guys have used/use launchd.
You can read the purpose and goals of systemd at http://0pointer.