The first process run, and thus the (grand)-parent of all others in a *nix system; also the only one which can never be stopped. Used to control boot/system services.
The current init daemon used on Raspbian and most other forms of GNU/Linux is systemd. However, the first version of Raspbian (7, "wheezy", following Debian's numbering) used SysV init, which dates back to the 1980's and was long used on a range of Unix derived operating systems.
Init is used to start system services at boot and control them thereafter.
