The term cloud computing itself can be confusing, as it was coined after the establishment of one of its key components, software-as-a service (SaaS). Research group Gartner's definition is of a style of computing where massively scalable solutions are provided as a service using Internet technologies.
Cloud computing comes in three main formats:
Infrastructure-as-a-service provides basic access to server hardware, storage and bandwidth. This is akin to the common model of outsourced applications hosting.
The next level is platform-as-a-service, which may also provide the basic operating software, as well as some add-on services such as database access, user interface, message queues, application servers and possibly a payment service.
This model is designed for companies that wish to host their own software and make it available to users, sometimes on a commercial basis. There are entire software businesses that are now run in this way, without ever selling packaged software.
The third model is software-as-a-service (SaaS), and provides customers with access to complete software applications, which are often integrated with others to provide greater functionality for the customer.
A final layer is one that actually offers workers as part of its service, such as Amazon's Mechanical Turk, which gives businesses access to an on-demand, scalable workforce, or TopCoder, which offers a service for writing computer code as a cloud service, backed by 200,000 independent software coders who bid for work. -- B Howarth
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