Standard SSL certificates are issued for one single domain name, e.g. www.yourdomain.xxx OR yourdomain.xxx .
 

Wildcard SSL certificates make it possible to use your certificate on an unlimited number of subdomains (e.g. https://order.yourdomain.xxx, https://my.yourdomain.xxx,  https://webmail.yourdomain.xxx and https://www.yourdomain.xxx OR https://yourdomain.xxx).

Note:
Standard SSL Certificate issued  for domain as yourdomain.xxx (without "www") will not work as www.yourdomain.xxx (with "www") and vice-versa. To use SSL certificate with both yourdomain.xxx and www.yourdomain.xxx you will need WildCard SSL Certificate.