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.de/blog/projects/systemd.html - while I don’t want to re-iterate the excellent documentation provided by the author, a few things to add is that systemd promises a lot while its ideas and concepts are not necessary new ;-)
To try systemd, you may use this forked Fedora 13 qemu image http://surfsite.org/f13-systemd-livecd.torrent (keep in mind, this version still used SysV, thus does not offer full socket and bus-based parallelization as stated in the project’s information page http://0pointer.de/blog/projects/systemd.html).
Fedora 14 is also planned to use systemd as a replacement to Upstart.