The public cloud (or external cloud) is freely available for access. You can use a public cloud to collect data of the purchasing of items from a Web site on the Internet. You can also use public cloud for the reasons, which are given as follows:
• Helps when an application is to be used by a large number of people, such as an e-mail application, on the Internet.
• Helps when you want to test the application and also needs to develop the application code.
• Helps when you want to implement the security for the application.
• Helps when you want to increase the computing capacity.
• Helps when you are working on the projects in collaboration.
• Helps when you are developing the project on an ad-hoc basis by using PaaS.

The private cloud allows the usage of services by a single client on a private network. The benefits of this model are data security, corporate governance, and reliability concerns. The private cloud is used by the organization when it has a huge, well-run data center having a lot of spare capacity. It is also used when an organization is providing IT services to its clients and the data of organization is highly important. It is best suited when the requirements are critical.

The characteristics of this model are given as follows:
• Provides capability to internal users and allows provision of services.
• Automates the tasks of management and provides the billing of consumption of a particular service.
• Offers a well-managed environment.
• Enables the optimization of computational resources, such as servers.
• Manages the workload of the hardware.
• Offers self-service based provisioning of hardware resources and software.

The Windows Azure operating system is used for running cloud services on the Windows Azure platform, as it includes necessary features for hosting your services in the cloud. It also provides runtime environment that consists of Web server, computational services, basic storage, queues, management services, and load balancers. The operating system provides development. Fabric for development and testing of services before their deployment on the Windows Azure in the cloud.

Some of the advantages of cloud service are given as follows:
• Helps in the utilization of investment in the corporate sector; and therefore, is cost saving.
• Helps in the developing scalable and robust applications. Previously, the scaling took months, but now, scaling takes less time.
• Helps in saving time in terms of deployment and maintenance.