init is the first process that is started by an Unix operating system. It has PID 1 and handles creation of all other processes and daemon processes required by the OS. It acts as the ancestor of all processes. There are many init systems that have been used in Linux distributions over the years. Some of the popular init systems are the classic System V init, upstart (Ubuntu used to use this) and systemd (popular right now for all Linux distros).
So, you are sitting at a Linux computer, do you know which init system it is using? There is no straightforward method to find this out.
Here are a few methods, one of which should work:
$ init --version init (upstart 1.12.1) Copyright (C) 2006-2014 Canonical Ltd., 2011 Scott James Remnant
This was on a Ubuntu 14.04 system. You can see that it uses upstart.
/sbin/initis a symbolic link to the actual init:
$ stat /sbin/init File: '/sbin/init' -> '/lib/systemd/systemd'
This was on a Ubuntu 16.04 system. You can see that it uses systemd.