Ubuntu has an init system for starting various services on boot, depending on the runlevel that is booted into. Apache by default adds itself to the various
/etc/rc*.d/ when you install it through apt, so it gets started by default for most runlevels.
To remove the startup links to the init script for apache2, you'll want to run something like:
update-rc.d apache2 disable
This should give you an output like:
Disabling system startup links for /etc/init.d/apache2 ...
Removing any system startup links for /etc/init.d/apache2 ...
And now apache won't start on boot. You can still start it manually using the script in
To see what services are currently enabled for boot-time startup, you can use a tool like chkconfig, which I don't believe is installed by default but is available in the repositories.
edit: if you don't want apache running at all, you might be better off just uninstalling it.