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

Public Cloud: The public cloud is defined as computing services offered by third-party providers over the public Internet, making them available to anyone who wants to use or purchase them

Private Cloud: This also offers activities and functions “as a service” but is deployed over a company intranet or hosted datacenter.

An example of a private cloud is where you maintain your own servers and infrastructure that hosts your applications and data.

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.