My understanding is that a shared SSL certificate will make the address of a page look something like https://yourdomainname.com.yourhostingcompany..... whereas the private SSL removes the hosting company from the address. A private SSL requires a dedicated IP so your account would need to upgrade to that.

Verisign is a well known company that issues the certificates and you pay extra for the goodwill that their name implies. You can get a private SSL for as little as $10 per year if you know how to install it yourself. Hostgator sells one and installs it for $50 which also comes with an image to let visitors know that their information is secure. I guess the question is whether having Verisign would result in more sales. Maybe others here have seen data on this.