what is cloud computing?

Cloud Computing

cloud is a computer server somewhere in internet   

In practice Cloud Computing is the use of a network of remote servers hosted on the Internet to store, manage, and process data, rather than a local server or a personal computer. In other words, cloud computing means storing and accessing data and programs over the Internet instead of your computer's hard drive. The cloud is just a metaphor for the Internet. All of us, either as an individual or as a company are already using the Cloud. For example, your mobile phone service, your emails, eBay, Amazon and may other applications are already running in internet. And when you want to access them, you have to connect to the internet using one of your gadgets.
Consumer vs. Business
There is an entirely different "cloud" when it comes to business. Some businesses choose to implement Software-as-a-Service (SaaS), where the business subscribes to an application it accesses over the Internet. (Think Salesforce.com.) There's also Platform-as-a-Service (PaaS), where a business can create its own custom applications for use by all in the company. And don't forget the mighty Infrastructure-as-a-Service (IaaS), where players like Amazon, Microsoft, Google, and Rackspace provide a backbone that can be "rented out" by other companies. (For example, Netflix provides services to you because it's a customer of the cloud services at Amazon.) of course, cloud computing is big business: The market generated $100 billion a year in 2012, which could be $127 billion by 2017 and $500 billion by 2020.
Common Cloud Examples
The lines between local computing and cloud computing sometimes get very, very blurry. That's because the cloud is part of almost everything on our computers these days. You can easily have a local piece of software (for instance, Microsoft Office 365) that utilizes a form of cloud computing for storage (Microsoft OneDrive). That said, Microsoft also offers a set of Web-based apps, Office Online, that are Internet-only versions of Word, Excel, PowerPoint, and OneNote accessed via your Web browser without installing anything. That makes them a version of cloud computing (Web-based=cloud).   

Google Drive
This is a pure cloud computing service, with all the storage found online so it can work with the cloud apps: Google Docs, Google Sheets, and Google Slides. Drive is also available on more than just desktop computers; you can use it on tablets like the iPad or on smartphones, and there are separate apps for Docs and Sheets, as well. In fact, most of Google's services could be considered cloud computing: Gmail, Google Calendar, Google Maps, and so on.

Apple iCloud
Apple's cloud service is primarily used for online storage, backup, and synchronization of your mail, contacts, calendar, and more. All the data you need is available to you on your iOS, Mac OS, or Windows device (Windows users have to install the iCloud control panel). Naturally, Apple won't be outdone by rivals: it offers cloud-based versions of its word processor (Pages), spreadsheet (Numbers), and presentations (Keynote) for use by any iCloud subscriber. iCloud is also the place iPhone users go to utilize the Find My iPhone feature that's all important when the handset goes missing.

Amazon Cloud Drive
Storage at the big retailer is mainly for music, preferably MP3s that you purchase from Amazon, and images—if you have Amazon Prime, you get unlimited image storage. Amazon Cloud Drive also holds anything you buy for the Kindle. It's essentially storage for anything digital you'd buy from Amazon, baked into all its products and services. Synchronization is a cornerstone of the cloud computing experience, even if you do access the file locally.
Public Cloud vs Private Cloud vs Hybrid Cloud
So far everything we have mentioned have been public cloud. In other words, companies such as Apple, Amazon, Microsoft, Google and Rack Space provide a platform in the Internet so that everyone can use some services for a fee or some other services for free.

Private cloud is what a company creates for itself in the Internet only for the exclusive use of its employees. This can be setup privately if the company can afford to pay for the hardware and a server house where they can host/place their servers. Then they create a website which only give access to the employees using Username/Password. Some private cloud services can be made on the Public cloud. For example a company can setup their entire network in the Internet using Windows Azure which is hosted by Microsoft. It will be used for exclusive use of the company employees, but the company will have to pay a subscription fee to Microsoft. Another example would be Office 365 or Exchange in Cloud.

The Hybrid model is a combination of Public and Private cloud. In this model a company decides to have its network running on the systems that are placed locally in their own premises, but at the same time move some services into the cloud such as Exchange or SharePoint. It must be said that Google and Amazon also provide a platform for companies to run private or hybrid Cloud, for example AWS from Amazon.