Definition and Essential Characteristics of Cloud Computing

Cloud computing, also known as "the cloud" is the definition of delivery of on-demand computing resources.

The US National institute of standards and technology defines cloud computing as a model for enabling convenient, on-demand network access to a shared pool of configurable computing resources that can be rapidly provisioned and release with minimal management effort or service provider interaction. Examples of clouds are:

• Network 
• Servers
• Storage applications 
• Services 

The cloud model is made of:

• 5 essential characteristics 
• 3 deployment models 
• 3 service models 

Let's talk first about the 5 essential characteristics:

1. On demand self-service: you can get access to the resources in the cloud, such as processing power, storage and network you need by using a simple interface, without requiring human interaction with each service provider 
2. Board network access: the resources on the cloud can be easily accessed via network through standard mechanisms and platforms like mobile phones, tablets, laptops and workstations.
3. Resource pooling: this give to the cloud service providers the economies of scales, which they pass on to their costumers, making cloud cost-efficient. All the resources are assigned and reassigned based on demand, without costumers needing to concern about the physical location of them.
4. Rapid elasticity: you can access the resources when you need them, and scale back when you don't, this mean the resources are elastically provisions and released.
5. Measured service: you only pay for what u use or reserve as you go. Resource usage is monitored, measured and reported transparently based on utilization.

So we can see, cloud computing is all about using technology "as a service" (CcaaS) 

It is a revolution because it changes the way the world consumes compute services by making them more cost-efficient, while also making organizations more agile in responding to changes in the market.

Now let's talk abut the 3 deployment models:

1. Public: Leverage cloud service over the open internet on hardware owned by the cloud provider, but its usage is shared by other companies.
2. Hybrid: using a mix of both, working together seamlessly
3. Private: the cloud infrastructure is provisioned for exclusive use by a single organization.it could run on-premises, or it could be owned, managed and operated by a service provider

And finally, the 3 service models: three layers in a computing stack

1. Infrastructure (IaaS): "infrastructure as a service model" you get access to infrastructure and physical computing resources such as services, networking, storage and data center space without the need to manage or operate them.
2. Platform (PaaS): "platform as a service model" you get access to the platform, that is hardware and software tools, usually those needed to develop and deploy applications to users over the internet. 
3. Application (SaaS): "Software as a service model" is a licensing and delivery model in which software and applications are centrally hasted and licensed on subscription basis, and sometimes also referred to as "on-demand software"

This information is based on the course "introduction to the cloud" available on cognitiveclass.ai