Cloud computing is a general term for anything that involves delivering hosted services over the Internet. These services are broadly divided into three categories: Infrastructure-as-a-Service (IaaS), Platform-as-a-Service (PaaS) and Software-as-a-Service (SaaS). The name cloud computing was inspired by the cloud symbol that's often used to represent the Internet in flowcharts and diagrams.
A cloud service has three distinct characteristics that differentiate it from traditional hosting. It is sold on demand, typically by the minute or the hour; it is elastic -- a user can have as much or as little of a service as they want at any given time; and the service is fully managed by the provider (the consumer needs nothing but a personal computer and Internet access). Significant innovations in virtualization and distributed computing, as well as improved access to high-speed Internet and a weak economy, have accelerated interest in cloud computing.
A cloud can be private or public. A public
cloud sells services to anyone on the Internet. (Currently, Amazon Web Services is the largest
public cloud provider.) A private cloud is a
proprietary network or a data center that supplies hosted services to a limited number of people.
When a service provider uses public cloud resources to create their private cloud, the result is
called a virtual private cloud. Private or public, the goal of cloud computing is to provide easy,
scalable access to computing resources and IT services.
Infrastructure-as-a-Service like Amazon Web Services provides virtual server instanceAPI) to
start, stop, access and configure their virtual servers and storage. In the enterprise, cloud
computing allows a company to pay for only as much capacity as is needed, and bring more online as
soon as required. Because this pay-for-what-you-use model resembles the way electricity, fuel and
water are consumed, it's sometimes referred to as utility computing.
Platform-as-a-service in the cloud is defined as a set of software and product development tools
hosted on the provider's infrastructure. Developers create applications on the provider's platform
over the Internet. PaaS providers may use APIs, website portals
software installed on the customer's computer. Force.com, (an outgrowth of Salesforce.com) and
GoogleApps are examples of PaaS. Developers need to know that currently, there are not standards
for interoperability or data portability in the cloud. Some providers will not allow software
created by their customers to be moved off the provider's platform.
In the software-as-a-service cloud model, the vendor supplies the hardware infrastructure, the
software product and interacts with the user through a front-end portal. SaaS is a very broad
market. Services can be anything from Web-based email to inventory control and database processing.
Because the service provider hosts both the application and the data, the end user is free to use
the service from anywhere.