First, it wasn't a typo to label this section as systemd in all lowercase letters. It is one of the mandates of Linux that any daemon, which is what systemd is, should be in lowercase. Also, daemons must also come with the lowercase suffix: d.
So what is systemd exactly? Briefly, it is a background process and a system management daemon for managing user space system services. Systemd will become standard plumbing for most major Linux distros and is an integral part of the BBB Debian distribution. It is the first process initiated on bootup in the Linux user space and the last process to terminate on shutdown. Basically, systemd starts the show on your Linux OS and turns out the lights as it's leaving.
Sound familiar? If it does, that's because systemd is the heir to init: Linux's longtime daemon of daemons.
Mastering systemd on your BBB helps us better understand system dependencies because systemd serves as a robust framework for these dependencies. Here are a couple of examples...