Once all modifications have been made, and the appropriate sites or modules enabled, it is best to test the configuration for obvious errors. While it is possible to use the init script in /etc/init .d/apache2 for this, the apache2ctl utility is specifically intended for, and better suited for this purpose. There is a configtest command option (detailed in the apache2ctl manual page) which will perform basic checking on all of the included files. It won't catch all errors, but it will catch the majority of them. After you are confident the configuration is correct, it is necessary to reload or restart Apache in order for the changes to become effective. Again, while the init script can handle this, using apache2ctl is the preferred method.
There are two ways to get Apache to reload its configuration files, a regular restart, or a graceful restart (which doesn't actually restart, it just reloads the configuration). The graceful restart is the preferred method because if there is some issue with the configuration, especially something the configtest was unable to catch, the web service continues running with the old configuration, avoiding server downtime while you fix the problem. A graceful restart may take some time if there are long-running threads, since it waits until all threads are idle before reloading the configuration. A full restart forcibly terminates all threads and reloads the configuration, and won't restart the server after it is fully stopped, if there are configuration errors.
