Systemd is an implementation of an init system, with some extras too.<p>Init systems are traditionally the first thing that is executed when the kernel boots and passes control to userspace. They're responsible for starting all the services that are configured/enabled.<p>In addition to this "basic" functionality systemd has extras which allow it to handle dependencies, start services on-demand, and handle logging, etc.<p>Beyond that you'd be best reading the documentation on the systemd site - I've never used it, but the basics don't seem too hard to understand.