The National Institute of Standards and Technology (NIST) gives the following definition of cloud computing:

Cloud computing is a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources (e.g., networks, servers, storage, applications, and services) that can be rapidly provisioned and released with minimal management effort or service provider interaction.

There are several other broadly accepted definitions of cloud computing but, all of them share the same features:

• Users should be able to provision and release resources on-demand.
• The resources can be scaled un and down automatically, depending on the load.
• The provisiones resources should be accessible over a network.
• Cloud service providers should enable pay-as-you-go model, where customers are charged based on the type and quantum of resources they consume.

We can find three types of clouds in cloud computing:

• Public cloud: In a public cloud, third-party service providers make resources and services available to their customers via the internet. The customers’ applications and data are deployed on infrastructure owned and secured by the service provider.
• Private cloud: A private cloud provides many of the same benefits of a public cloud but the services and data are managed by the organization or a third-party, solely for the customer’s organization. Usually, private cloud places increase administrative overheads on the customer but gives greater control over the infrastructure and reduce security-related concerns. The infrastructure may be located on or off the organization’s premises.
• Hybrid cloud: A hybrid cloud is a combination of both a private and a public cloud. The decision on what runs on the private versus the public cloud is usually based on several factors, including business criticality of the application, sensitivity of the data, industry certi cations and standards required, regulations, and many more. But in some cases, spikes in demand for resources are also handled in the public cloud.

 As well, we can find three service models:

• Infrastructure as a Service (IaaS): IaaS provides users the capability to provision processing, storage, and network resources on demand. The customers deploy and run their own applications on these resources. Using this service model is closest to the traditional in-premise models and the virtual server provisioning models (typically offered by data center outsourcers). The bonus of administering these resources rests largely with the customer.
• Platform as a Service (PaaS): The service provider makes certain core components, such as databases, queues, workflow engines, e-mails, and so on, which are available as services to the customer. The customer then leverages these components for building their own applications. The service provider ensures high service levels, and is responsible for scalability, high- availability, and so on for these components. This allows customers to focus a lot more on their application’s functionality. However, this model also leads to application-level dependency on the providers’ services.
• Software as a Service (SaaS): Typically, third-party providers using a subscription model provide end-user applications to their customers. The customers might have some administrative capability at the application level, for example, to create and manage their users. Such applications also provide some degree of customizability, for example, the customers can use their own corporate logos, colors, and many more. Applications that have a very wide user base most often operate in a self-service mode. In contrast, the provider provisions the application for the customer for more specialized applications. The provider also hands over certain application administrative tasks to the customer’s application administrator (in most cases, this is limited to creating new users, managing passwords, and so on through well-defined application interfaces).

From an infrastructure perspective, the customer does not manage or control the underlying cloud infrastructure in all three service models.