What is a Wildcard SSL certificate?

A wildcard SSL certificate is used to secure unlimited sub domains that share a common base domain. For example, if your base domain is 'books.com', you can secure sales.books.com, www.books.com, secure.books.com, and shop.books.com with one wildcard certificate.

How do I use a wildcard SSL certificate in IIS?

Say you have five sites in IIS where you want to use one wildcard certificate. You'd pick one of the sites and generate a CSR in IIS for that site using *.yourdomain.com as the Common Name field. When the order is complete and the wildcard certificate is sent to you by email, you go back to that site in IIS and import the wildcard certificate. Then, using the Web Server Certificate Wizard in IIS, go to each of the other four sites and assign an existing certificate to each of the remaining four sites. Note that each site using the wildcard SSL certificate must have its own IP.

How many sub domains can I secure with a wildcard certificate?

Unlimited, as long as each sub domain shares the same common base domain. If your base domain is 'books.com', you can secure sales.books.com, www.books.com, secure.books.com, and shop.books.com with one wildcard certificate.