Additional to the historical ones (SysV and BSD), systemd recently added another init system to the Unix world.
An objective comparison of these systems would be nice. If there are other major ones, add them, e.g. upstart, launchd. Not in the sense of showing their features side-by-side, but simply describing how they work and how you do things yourself.

In the end, you should have created a small article that enables anyone using one of these systems to switch to the other one just by reading this article.

Our preferred format for the submission is some form of plain text, as that is most portable and can be read anywhere.  If you want to use a word processor, please export the file as text.  You could also add something like markdown or latex to add structure (these are both plain text based), but that is not a requirement.