Re: What is systemd
Others have done a good explanation of what a Unix Init system does. I believe they miss a couple of valuable points.
Firstly, systemd came about to try to improve boot speed. Because a typical Unix init system is made up of a series of shell scripts, you have to start a shell, parse the script, then run it for each init task. Shells can be relatively expensive to start, so systemd avoids this by not using a shell, and just having a single binary that reads configuration files.
An objection to systemd, though, is that systemd is more than just an init system and actually does lots of other things that are typically done by separate little programs. (So breaking the Unix tenant of "Do one thing and do it well") By taking on more and more little tasks, it becomes more and more tied to Linux - you can't port systemd easily to the BSDs for example. As other things start to depend on systemd, then they too become impossible to port to other versions of Unix. (There is actually a project trying to create/port a minimalist version of systemd which is more cross-unix friendly.)
Even on lwn.net (which is usually quite tame compared to other forums) the discussion about the whole Systemd/Debian debate is getting quite heated. The article about the Devuan fork has already attracted over 200 posts in two days - quite a high comment rate for an lwn article.