If you change a configuration file for a system service, it is usually necessary to stop and restart the service to make it read the new configuration. If you are reconfiguring the X server, it is often convenient to change from runlevel 5 to runlevel 3 to make testing easier and then switch back to runlevel 5 to re-enable the graphical login. If a service is improperly configured, it is easier to stop and restart it until you have it configured correctly than it is to reboot the entire machine.
sudo /etc/init.d/apache2 start
Starting apache 2.0 web server [ OK ]
The script will execute the proper program(s) and report the status of it. Stopping services is equally easy, using the stop keyword.