Restart httpd after changes in the httpd.conf

Yes. HTTPD.conf is read on apache start-up, so for any changes to take affect you need to restart it.

Yes you need to restart the server by

sudo service httpd restart

You can check errors in your config files by

apachectl -t

Changes may take some time to reflect.

You can have apache re-read its config files without restarting by sudo apachectl graceful (or apache2ctl on debian).

From the Apache docs:

Gracefully restarts the Apache httpd daemon. If the daemon is not running, it is started. This differs from a normal restart in that currently open connections are not aborted. [...] This command automatically checks the configuration files as in configtest before initiating the restart to make sure Apache doesn't die.