A public cloud is an infrastructure delivered over the internet by a cloud provider who makes computing resources, such as processing power, memory or storage, publicly available over the internet. These resources are hosted on the provider’s servers in one or many data centres located all over the world.
The infrastructure is multi tenancy which means it is shared among other customers.
Good examples of public clouds are free services such as web-based email from Google or Microsoft. In 2006 Amazon became the world's largest provider of public cloud computing. It assessed that a large part of its global computing infrastructure was unused and therefore made a strategic decision to set up a new entity called Amazon Web Services (AWS) dedicated to renting out its spare capacity over the Net.
This option offers the most flexibility as you are only charged for the resources that you use on a pay-as-you-go basis but doesn’t have the security offered by private cloud.