systemd is the new system startup and service manager for Linux, replacing the old SysV init. Forget everything you ever knew about how Linux systems start up and manage services. systemd is differentand you need to understand how to work with it.
According to the hype, systemd "provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux cgroups, supports snapshotting and restoring of the system state, maintains mount and automount points and implements an elaborate transactional dependency-based service control logic." It also promotes run-on sentences and excessive fervor in its adherents.
Come see what systemd is about so you can avoid being drowned when it hits a Linux system near you.