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A public cloud is a platform that uses the standardcloud computingmodel to make resources, such as virtual machines (VMs), applications or storage, available to users remotely. Public cloud services may be free or offered through a variety of subscription or on-demand pricing schemes, including a pay-per-usage model.

The main benefits of public cloud service are:

While the concept of cloud computing has been around since the 1960s, it didnt reach public popularity for enterprises until the 1990s. Salesforce, now a top software as a service (SaaS) provider, entered the market in 1999 by delivering applications through a website. It was soon followed by browser-based applications, such as G Suite, that could be accessed by numerous users.

In 2006, the retail company Amazon launched Elastic Compute Cloud (EC2), its infrastructure as a service (IaaS) platform, for public use. Under its cloud division, Amazon Web Services (AWS), enterprises could "rent" virtual computers but use their own systems and apps. Soon after, Google released Google App Engine, its platform as a service (PaaS) service, for application developed and Microsoft came out with Azure, also a PaaS offering. Overtime, all three built IaaS, PaaS and SaaS offerings. Even legacy hardware vendors entered the market, such as IBM and Oracle.

However, not all vendors that tried to compete succeeded. Verizon, HPE, Dell, VMware and others were forced to shut down their public clouds, often have refocused on hybrid cloud.

A public cloud is a fully virtualized environment. In addition, providers have a multi-tenant architecture that enables users -- or tenants -- to share computing resources. Each tenant's data in the public cloud, however, remains isolated from other tenants. Public cloud also relies on high-bandwidth network connectivity to rapidly transmit data.

Public cloud storage is typically redundant, using multiple data centers and careful replication of file versions. This characteristic has given it a reputation for resiliency.

Public cloud architecture can be further categorized by service model. Common service models include:

The termpublic cloudarose to differentiate between the standard cloud computing model and private cloud, which is a proprietary cloud computing architecture dedicated to a single organization. Private cloud differs from public cloud, as it serves as an extension of a company's existing data center and is accessible only by that company.

A third model, hybrid cloud, is maintained by both internal and external providers. In effect, a hybrid cloud is a combination of public and private cloud services, with orchestration between the two. In some cases, this model is attractive because it enables organizations to tap into the benefits of the public cloud, while maintaining their own private cloud for sensitive, critical or highly regulated data and applications. The forth option is a multi-cloud architecture in which an enterprise uses more than one cloud. Most often it refers to the use of multiple public clouds.

In general, the public cloud is seen as a way for enterprises to scale IT resources on demand, without having to maintain as many infrastructure components, applications or development resources in house.

Thepay-per-usagepricing structure offered by most public cloud providers is also seen by some enterprises as an attractive and more flexible financial model. For example, organizations account for their public cloud service as an operational or variable cost rather than capital or fixed costs. In some cases, this means organizations do not require lengthy reviews or advanced budget planning for public cloud decisions.

However, because users typically deploy public cloud-based services in aself-service model, some companies find it difficult to accurately track cloud service usage, and potentially end up paying for more cloud resources than they actually need. Some organizations also just prefer to directly supervise and manage their own on-premises IT resources, including servers.

Because of the multi-tenant nature of public cloud, security is an ongoing concern for some enterprises. Public cloud providers offer security services and technologies, such as encryption and identity and access management tools.

However, it is the enterprises responsibility to implement such offerings and use best practices to protect their data. A shared-responsibility model helps identify which components are the responsibility of the cloud vendor and which should be secured by the user.

Some organizations choose to keep workloads on premises -- especially those with strict regulatory or governance requirements.

The public cloud market is led by a few key players: AWS, Microsoft and Google. These providers deliver their services over the internet, or through dedicated connections, and use a fundamental pay-per-usage approach. Each provider offers a range of products oriented toward different workloads and enterprise needs.

Estimates of public cloud usage vary widely across different countries, but most market research and analyst firms expect continued growth in worldwide adoption and cloud revenues.

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What is a Public Cloud? - Definition from WhatIs.com