Older but very informative article by Carla Schroder about systemd. systemd is controversial for several reasons: It’s a replacement for something that a lot of Linux users don’t think needs to be replaced.
Red Hat is the inventor and primary booster of systemd, so the best distros for playing with it are Red Hat Enterprise Linux, RHEL clones like CentOS and Scientific Linux, and of course good ole Fedora Linux, which always ships with the latest, greatest, and bleeding-edgiest.
Essential subsystems have deep hooks in the kernel, all manner of management scripts, and software package dependencies, so replacing one is not a trivial task
The article then explains how /etc/inittab is no more. Instead, we have a /etc/systemd/system/ directory chock-full of symlinks to files in /usr/lib/systemd/system/. /usr/lib/systemd/system/ contains init scripts; to start a service at boot it must be linked to /etc/systemd/system/.
Cheatsheet, links to external resources and more information in the article. systemd has 12 unit types and it is fairly complex, so please read to learn more!
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