Standard Wildcards give one domain of your choice complete, full protection with an SSL Certificate. This will apply to both:
www.yourdomain.com
yourdomain.com
A single Wildcard SSL Certificate can be shared across various subdomains of your site, giving them all maximum protection without having to have multiple SSL's. For example, a Wildcard SSL Certificate would cover:
portfolio.yourdomain.com
blog.yourdomain.com etc
As well as the www. version of the address