What is wildcard certificate? The wildcard SSL is another type of secure certificates that helps to enable the SSL encryption on your several sub domains with the use of one certificate, unless the domains are still in control by the similar organization and they are sharing same domain second-level name. Like, for example, a wildcard certificate released to SSL Company using the same name like “SSL.com” will be used to keep it secure with the following possible domains such as “login.ssl.com”, “support.ssl.com” or “payment.ssl.com” and more. A wildcard notification consists of asterisk then a period before the chosen domain name. These are wildcards to broaden the SSL encryption to its sub domains. In case of www.ssl.com as example, the *.ssl.com will also secure its other sub domains such as “login.ssl.com” and more.
see http://info.ssl.com/article.aspx?id=12154&cNode=8P3K3M for more details