update-rc.d is the debian way of managing services and daemons started by the init scripts. defaults sets up services to start and stop at certain points when changing runlevels. For ssh update-rc.d ssh defaults is the same as typing: update-rc.d ssh start 20 2 3 4 5 . stop 20 0 1 6 .
man update-rc.d will give you a better explanation.
ls -l /etc/rc5.d shows you what services are started and stopped and in which order at the graphical runlevel.