Depending on the audience and security, cloud computing can be organized into four groups.
Public cloud is an IT capability that is rendered as a service to any consumer over the public Internet. Some of the well known examples are Saleforce.com, Google App Engine, Microsoft Azure, and Amazon EC2.
Private cloud is an IT capability that is rendered as a service to a select group of consumers. Private cloud service provider could be the IT department of the same organization as the customer. It could also be a third party vendor who has created a network for specific use of that customer. Note that the network on which the service is provided could be either a public network such as the Internet or a private network. Examples include hospitals or universities that organize a holding company to purchase infrastructure and build cloud services for their consumption.
Internal cloud is a subset of a private cloud. It is an IT capability rendered as a service by an IT department to its own business customers. Example would be that an IT department building a highly virtualized environment for consumption of internal application developers. This would automate the procurement and provision process. It would also automate internal charge back to the developer’s or the project cost center. The role of the IT environment team would be to monitor and provision the hardware, middleware, and application licensing so that the demand does not overrun capacity.
External cloud is an IT capability that is rendered as a service to a business that is not hosted by its own IT organization. An external cloud can be public or private, but must be implemented by a third party.