What is Public Cloud

A public cloud is one based on the standard cloud computing model, in which a service provider makes resources, such as applications and storage, available to the general public over the Internet. Public cloud services may be free or offered on a pay-per-usage model.
  The main benefits of using a public cloud service are:

• Easy and inexpensive set-up because hardware, application and bandwidth costs are covered by the provider.
• Scalability to meet needs.
• No wasted resources because you pay for what you use.

The term "public cloud" arose to differentiate between the standard model and the private cloud, which is a proprietary network or data center that uses cloud computing technologies, such as virtualization. A private cloud is managed by the organization it serves. A third model, the hybrid cloud, is maintained by both internal and external providers.
Examples of public clouds include Amazon Elastic Compute Cloud (EC2), IBM's Blue Cloud, Sun Cloud, Google AppEngine and Windows Azure Services Platform.

Ref: http://searchcloudcomputing.techtarget.com/definition/public-cloud