Because the traditional System V init daemon (SysVinit) does not deal well with modern hardware, including hotplug devices, USB hard and flash drives, and network-mounted filesystems, Ubuntu replaced it with the Upstart init daemon.
This article is excerpted from the recently published book A Practical Guide to Ubuntu Linux.
Several other replacements for SysVinit are also available. One of the most prominent, initng, is available for Debian and runs on Ubuntu. Solaris uses SMF (Service Management Facility) and Mac OS uses launchd. Over time, Ubuntu will likely come to incorporate features of each of these systems into Upstart.