Typically this will involve checking and mounting the partitions and drives that form the filesystem, loading drivers for connected devices and starting the X windowing system or other login environment.
In the default Upstart configuration, the primary task run on the startup event is the /etc/init/rc-sysinit.conf job responsible for generating the System V compatible runlevel(7) event. See that page for a more detailed explanation of this process.
Paradoxically there is currently no corresponding Upstart-native event signifying that the system is to be shutdown, only the System V compatible runlevel 0 and runlevel 6 events provide this functionality.
start on startup