Activate a vHost
$> a2ensite 000-ask-sheldon.com
In this example 000-ask-sheldon.com refers to the vhost configuration that is defined under /etc/apache2/sites-available/000-ask-sheldon.com .
a2ensite creates a symbolic link under /etc/apache2/sites-enabled with the name 000-ask-sheldon.com that points to the configuration file /etc/apache2/sites-available/000-ask-sheldon.com. So you can see, that you can do it manually too.
Deactivate a vHost
$> a2dissite 000-ask-sheldon.com
In contrast to the first example this command removes the symbolic /etc/apache2/sites-enabled/000-ask-sheldon.com. So the vHost is deactivated again.
Test configuration
To check if your server configuration is correct you can use the following command before restarting the webserver and indeed activate the recent changes:
$> apachectl configtest
If there are errors in your configuration (any), they will be shown.