What is systemd and what are its main functions?

systemd is the init system and service manager — PID 1, the first process the kernel starts; it boots the system and supervises every service.

systemd is the "parent of all processes". The kernel hands control to it as PID 1, and from there it brings the whole userspace up and keeps it running. It replaced the old SysV init scripts (default since RHEL/CentOS 7) and does far more than just start things in sequence.

Why it was a big change — its main jobs:

• Parallel startup: services with no dependency on each other start simultaneously, so boot is much faster than sequential scripts.
• Dependency management: units declare what they need (After=, Requires=), and systemd computes a correct start order instead of relying on hand-numbered scripts.
• On-demand (socket/D-Bus) activation: a service can stay off until its socket actually receives a connection, saving resources.
• Supervision & restart: if a service crashes, systemd can restart it automatically (Restart=on-failure).
• Unified logging via journald, and management of mounts, timers, and sockets — all as "units".

Where its config lives (later directories override earlier ones):

• /usr/lib/systemd/system/ — units shipped by packages
• /etc/systemd/system/ — your custom or overriding units

Tip: Almost everything you do with systemd goes through one command — systemctl.

Go deeper:

• systemd (Wikipedia) — PID 1, parallel startup, dependency/socket activation, supervision and journald.