Private CA is for organizations that do not care to be digitally recognized by outside orgnaizations. It does not have a trusted root and so is not ‘trusted’ by other organizations. It does not preclude you from doing busines with anyone it is just a means of authenticating your organization. A Public CA is a trusted CA and is lashed to public root which has certified you as a trusted CA. Trusted CAs are normally associated with internet financial transations. Go to VeriSign for do a lookup on Private/Public CA and it will explain it better.