SSL (Secure Socket Layers) encrypts information (like Social Security numbers, credit card numbers, or any other information entered into a web site) as it is transferred across the internet.

Shared SSL:
This is installed globally on the server and all clients can use it. It is configured to work with the server’s name. However, as this is registered on server and NOT your domain it will encrypt the data transmitted but it will warn the users that the domain being used does not match that registered in the certificate.

Private SSL:
This is a certificate issued specifically for your own domain name and will not cause any warnings messages.

A Private SSL would be preferred for protecting credit card information during purchasing for E-Commerce websites. This is because shared SSL warnings would make people feel uncomfortable with submitting their credit card information on your website when warnings are being generated by their browser.