systemd (formerly udev) is an open source system management daemon. In other words, it runs in the background and acts as a system and service manager for Linux-based operating systems.
Compatible with LSB and UNIX System V init scripts
Being compatible with the LSB (Linux Standard Base) and UNIX System V init scripts, systemd uses D-Bus and socket activation for starting services, and provides aggressive parallelization capabilities. In addition it supports restoring and snapshotting of the system state, maintains automount and mount points, keeps track of processes by using Linux control groups, offers on-demand starting of daemons, and implements a complex transactional dependency-based service control logic.
A drop-in replacement for sysvinit
systemd is included in almost every kernel-based Linux operating system, and it can be used as a drop-in replacement for the sysvinit software, but also for the inetd, acpid, atd, watchdog, cron, syslog, and pm-utils daemons. The program also comes with a built-in login manager, called systemd-logind, designed as a drop-in replacement for the deprecated ConsoleKit software. It features various multiseat improvements.
Can administer network configurations too
Since version 209, systemd can also administer network configurations, thanks to the integration of the networkd daemon. For example, it can statically assign IP addresses, as well as to provide basic bridging configuration.
Supported Linux distributions
Since its appearance, back in 2011, the project has been adopted quite fast by many popular Linux distributions, including Arch Linux, Fedora, Gentoo Linux, Mageia, openSUSE, Sabayon Linux, Frugalware Linux, Ångström, and CoreOS. In addition, many other powerful Linux operating system will adopt systemd in their forthcoming releases, such as Ubuntu 14.04 LTS (Trusty Tahr), Debian GNU/Linux 8 (Jessie), and Red Hat Enterprise Linux 7.
Even if some people tend to write SystemD, system d, system D or System D, the program’s name is spelled and written systemd. It is definitely the future of any Linux distribution, changing the way we interact with the operating system.