systemd is an alternative approach to SystemV and Upstart init daemons for Linux. It is intended to provide a better framework for expressing services' dependencies, allow more work to be done in parallel at system startup, and to reduce shell overhead.
systemd is a system and service manager for Linux, compatible with SysV and LSB init scripts. 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 can work as a drop-in replacement for sysVinit.
One of the most common questions about systemd on StackExchange is "Why doesn't my app work under systemd when I can run it directly?."
More here at http://0pointer.de/blog/projects/systemd.html#faqs