A normal SSL certificate secures a single domain name..
A Wildcard SSL certificates secures multiple sub-domains of a single domain name. For example, a single Wildcard certificate can secure www.yourdomain.com, blog.yourdomain.com, and mail.yourdomain.com.
Wildcard certificates secure the common name and all sub-domains at the level you specify when you submit your request. Just add an asterisk (*) in the sub-domain area to the left of the common name.