Dan,
An SSL certificate will work for the domain or "host record" for the domain that it was purchased for for the life of the certificate.
It can be moved between servers (same type of server, ie. Windows, or Apache, etc.) It's domain specific not page or application specific.
You generally get a certificate for the www. (the host record) record. You can get what is called a wildcard certificate (about 10 times the cost) that will take any host record inside your domain.
A couple of examples,
A standard "host record" certificate,
"www".yourdomain.com
This will enable https connections to
www.yourdomain.com. The pages can be simple html, asp, aspx etc., it doesn't matter. It can be a BVC2, BVC2004, BVC5 site etc., it doesn't matter what the page is, it only matters what the domain is.
A wildcard SSL certificate,
www.
ftp.
mail.
shop.
store.
forum.
etc. etc. etc. Basically any host record for your domain. Your forum can be on a server in our Somerville facility, your FTP can be in the Manchester facility, and your www in our Bedford facility, the same wildcard certificate will work on all three servers at the same time.
Hope that helps.