John McCarthy is sure to be called "the father of the idea of cloud computing" as he invented this concept along with the idea of "artificial intelligence" in 1960's. He also said that the time would come when most of the computer calculations will be made using so-called "public utilities".

Since 2000, thanks to the rapid development of communication channels and the rapidly growing user needs, the cloud systems have gained much popularity, and in 2006 the first public cloud service was announced by Amazon (it was called Amazon_Elastic_Compute_Cloud).

The concept of cloud services has long been associated with the metaphorical image of the Internet, through which some services are available. In simple terms, cloud computing is the provision of computer services (servers, storage, databases, network equipment, software, analytics, etc.) over the Internet. Companies that provide such computing services are called "providers of cloud services". Typically, they charge a service fee based on the same way as charging a fee for water or electricity at home utilities. There are also cloud-based services that have a certain free limit. As usual, they are all cloud file storage services, for example, Google Drive, DropBox, etc.

The cloud technology exists for more than 10 years by now, but many are still wondering what is cloud computing and what it is made for.

For what purposes we can use clouds?

There is a high chance, that you have been using cloud technologies for a long time, being unaware of them. Public mail services, Internet services for creating and editing documents, publishing and processing images, audio and video storage, online games - they are all mostly based on cloud technologies.

What service could be considered as a "cloud" one?

The National Institute of Standards and Technology in USA suggests the following official definition of cloud computing:

Cloud computing is a model for enabling ubiquitous, 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. This cloud model is composed of five essential characteristics, three service models, and four deployment models. (The NIST Definition of Cloud Computing, SP800-145.pdf)

• On-demand self-service. A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service provider.
• Broad network access. Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, tablets, laptops, and workstations).
• Resource pooling. The provider's computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand. There is a sense of location independence in that the customer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of resources include storage, processing, memory, and network bandwidth.
• Rapid elasticity. Capabilities can be elastically provisioned and released, in some cases automatically, to scale rapidly outward and inward commensurate with demand. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be appropriated in any quantity at any time.
• Measured service. Cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Typically this is done on a pay-per-use or charge-per-use basis. Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service.

So, Cloud Computing is measured, on-demand, elastic computing using pooled resources, usually on the Internet.

Cloud Computing Service Models

• Software as a Service (SaaS). The capability provided to the consumer is to use the provider's applications running on a cloud infrastructure2. The applications are accessible from various client devices through either a thin client interface, such as a web browser (e.g., web-based email), or a program interface. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, storage, or even individual application capabilities, with the possible exception of limited user-specific application configuration settings.More on Software as a Service (SaaS)
• Platform as a Service (PaaS). The capability provided to the consumer is to deploy onto the cloud infrastructure consumer-created or acquired applications created using programming languages, libraries, services, and tools supported by the provider. The consumer does not manage or control the underlying cloud infrastructure including network, servers, operating systems, or storage, but has control over the deployed applications and possibly configuration settings for the application-hosting environment. More on Platform as a Service (PaaS).
• Infrastructure as a Service (IaaS). The capability provided to the consumer is to provision processing, storage, networks, and other fundamental computing resources where the consumer is able to deploy and run arbitrary software, which can include operating systems and applications. The consumer does not manage or control the underlying cloud infrastructure but has control over operating systems, storage, and deployed applications; and possibly limited control of select networking components (e.g., host firewalls). More on Infrastructure as a Service (IaaS).

Although it is not part of the NIST Definition of Cloud Computing, Network as a Service (NaaS) is sometimes factored out of IaaS. See Network as a Service (NaaS).

These service models need illustrations an further discussion to make them easier to understand. You can find such illustrations and further discussion of these service models in Categories of Cloud Providers.

Cloud Computing Deployment Models

• Private cloud. The cloud infrastructure is provisioned for exclusive use by a single organization comprising multiple consumers (e.g., business units). It may be owned, managed, and operated by the organization, a third party, or some combination of them, and it may exist on or off premises. More on Private Clouds.
• Community cloud. The cloud infrastructure is provisioned for exclusive use by a specific community of consumers from organizations that have shared concerns (e.g., mission, security requirements, policy, and compliance considerations). It may be owned, managed, and operated by one or more of the organizations in the community, a third party, or some combination of them, and it may exist on or off premises. More on Community Clouds.
• Public cloud. The cloud infrastructure is provisioned for open use by the general public. It may be owned, managed, and operated by a business, academic, or government organization, or some combination of them. It exists on the premises of the cloud provider. More on Public Clouds.
• Hybrid cloud. The cloud infrastructure is a composition of two or more distinct cloud infrastructures (private, community, or public) that remain unique entities, but are bound together by standardized or proprietary technology that enables data and application portability (e.g., cloud bursting for load balancing between clouds). More on Hybrid Clouds.