Cloud infrastructure it’s about the hardware and software components that make a cloud functional.
All the components like servers, storage, networking and virtualization software that are needed to support the computing requirements of a cloud computing model.
In addition, cloud infrastructures include a software abstraction layer that virtualizes resources and logically presents them to users through programmatic means.

In cloud computing, virtualized resources are hosted by a service provider or IT department and delivered to users over a network or the Internet. These resources include virtual machines and components such as servers, compute, memory, network switches, firewalls, load balancers and storage.

In a cloud computing architecture, which refers to the front end and back end of a cloud computing environment, cloud infrastructure consists of the back end components.

Cloud infrastructure is present in each of the three main cloud computing models infrastructure as a service (IaaS), platform as a service (PaaS) and software as a service (SaaS). Together, these three models form what’s often called a cloud computing stack, with IaaS as the foundation, PaaS as the middle layer, and SaaS as the top layer.

Businesses use cloud infrastructures to run their applications. Unlike subscription-based pricing models, or payment structures that enable users to subscribe to vendor services for a set price, cloud infrastructures are typically purchased using a pay-per-use model. In a pay-per-usage model, users only pay for the services consumed generally on an hourly, weekly or monthly basis.

Rather than purchase cloud infrastructure from a provider, businesses can also build cloud infrastructures on-premises. When cloud providers maintain the cloud infrastructure, the environment is a public cloud. When the organization using cloud maintains the cloud infrastructure, the environment is a private cloud. And when both the cloud provider and user own pieces of the cloud infrastructure, the environment is a hybrid cloud.

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http://searchcloudcomputing.techtarget.com/definition/cloud-infrastructure