What is the difference between public cloud and private cloud? Is salesforce.com is a public cloud or private cloud?

Public Cloud: Could services are provided “as a service” over the Internet with little or no control over the underlying technology infrastructure. More than one tenant can use the same resources.

Private Cloud: This also offers activities and functions “as a service” but is deployed over a company intranet or hosted datacenter. This is private product for a company or organization offering advance security.

Salesforce.com: Is a public cloud as it is hosted on salesforce.com data centers and data of more than one tenant resides on same servers.