A Public Cloud is implemented using a shared data center infrastructure of hardware and software that is shared by multiple organizations. The data center is off-premises.
The Cloud Computing Stack in a Public Cloud is also shared with other organizations. The data, however, for each organization is kept separate. If the data center is shared but not the Cloud Computing Stack, that is a Virtual Private Cloud. If neither the data center nor the Cloud Computing Stack are shared, that is a Private Cloud.
A Public Cloud may participate in a Hybrid Cloud.
The following figure uses Customer Relationship Management (CRM) Software as a Service (SaaS) to illustrate a Public Cloud.
More on the general topic: Types of Clouds in Cloud Computing