init is the mother of all processes. It is started by the kernel when the system is first booted and has a pid of 1. The function of any init system is to create "userland". It runs scripts or (in the case of systemd) programs, that set the system clock, choose a console keyboard map and font, check and mount filesystems, get the network up and running, and finally provide users with a means to log in. Thereafter init continues to run in the background. It can be invoked at any time to change the run level and is finally called upon to shut down the system. It also acts as a parent for orphaned processes.
One of the oldest init systems is sysvinit. This was developed for Unix System V and was once the most widely used init system on Linux. However there are now a number of rival systems: bsdinit (originally developed for BSD Unix), runit, upstart and systemd.