Upstart is an open source drop-in replacement for the well known, yet deprecated initd (also known as System V init or /sbin/init daemon) software designed to start certains services and tasks during the boot process of a Linux operating system, administering them while the OS is in use, as well as to stop them during shutdown.
It's easily deployable in Linux distributions
The main difference between the init daemon and Upstart, is that the latter uses events to handle specific services during system startup and shutdown. It was mainly designed for and used in the Ubuntu Linux operating system.
However, the project is developed in such a way that it can be easily deployed and integrated in any other Linux distribution, as a replacement for System-V init. The sad part of this project is that it was only used in Ubuntu (and its derivatives) and Fedora, as other Linux distros switched to the more complex and modern systemd software.
Handles services and tasks
As mentioned, Upstart can handle both services and tasks during boot or shutdown processes based on events, which can be generated as either services or tasks. In addition, it can respawn dead services, communicate with the init daemon via the D-Bus service, and handle user services.
It can receive events from any other process, respawn and supervise init daemons that are separated from their parent process, and allows users to stop or start their own user services.
Availability and supported OSes
The project is distributed only as a source archive, which can be configured, compiled and installed in any Linux operating system. However, the developers recommend users to use the official Upstart packages from the default software repositories of their Linux distribution.
Apparently, systemd has received so much attention from Linux OS developers that even Ubuntu will switch to it, and that says a lot about the future of the Upstart project.