Different Types of Cloud Computing
Cloud computing has grown rapidly over the last few years. With that growth,has come diversity in the number of different configurations available and in the terms used to describe them. However, standards organizations have also been tracking the rise of cloud computing. They now offer more precise definitions and categories to help both service providers and customers see eye to eye. NIST (the US National Institute of Standards and Technology) has created a definition of cloud computing and based on common features, what the cloud service offers, and how that service is provided. 
Common Features of Cloud Computing Before diving into the differences, NIST suggests a list of essential characteristics to confirm that a service is truly a cloud service.
Pay as You Go Self-Service. Without needing to consult or negotiate with the service provider, as a customer you can decide your level of consumption of the online computing resources.
Resource Elasticity. You can change the amount of resources you use at any time, without any upper limit.
Appropriate Service Billing. Cloud computing services are charged for on the basis of appropriate units (disk space, processor time or user accounts, for instance).
Resource Sharing. You typically share a physical machine with other users, who may come and go according to their cloud computing needs.
Flexible Network Access. Internet is widely used for customers to connect to the cloud computing service.
Differences in the Levels of cloud computing service The NIST model defines three levels of service: IaaS, PaaS and SaaS.
IaaS (Infrastructure as a Service) is the most basic level and provides online computing resources like storage space and processing power.
PaaS (Platform as a Service) at the middle level provides computing resources where the operating system is already installed and other items such as a database server and an application development environment are also available.
SaaS (Software as a Service), the highest level of service, is where the customer uses specific online applications (online accounting and email programs are two examples).
Choices in How the Cloud Service is Deployed The online service offered by the cloud computing provider may be made available in any of the ways below.
A public cloud service is available for anyone to use.
A private cloud service is usually operated on behalf of just one organization.
A community cloud service serves a particular group of customers, for example, all the state health organizations in a given region.
A hybrid cloud combines two or more of the above, usually with flexibility to expand from one to another if demand for computing resources suddenly rises.
The geographical location of the services may vary (a private cloud may be operated on the customer’s own premises
Access to the service may be via a shared network or a dedicated connection.
And the Rest? The model above is convenient for describing many of the current flavors of cloud computing on offer. However, there are a few more that have also been defined: Collaboration as a Service, Database as a Service, Desktop as a Service and Network as a Service are some examples. It’s also worth remembering that other forms of cloud computing have also been in existence for some time. They include client-server computing, mainframe computing (especially time-sharing services), peer-to-peer computing and online gaming, to name a few. While sharing the common features of the cloud computing model described above, some also offer yet further variations in the types of online service provided (interactive online gaming, for instance) and their deployment (such as peer-to-peer).
