iCloud is a cloud storage and cloud computing service from Apple. It was first announced on June 6, 2011 at the Apple World Wide Developer Conference (WWDC). The service which replaces Apple's MobileMe web service and acts as a data syncing center for email, contacts, calendars, bookmarks, notes, to-do lists and other data.To create an iCloud account your will require a device running iOS 5 or later or a Mac running OS X Lion. PC users who are running Windows Vista can also sync to iCloud through the iCloud control panel. The Windows features are limited, but you can sync your information from Outlook to iCloud.
The service is available for free for the first 5GB. Additional storage can be purchased in tiers of 10, 20 or 50GB for $20 per 10GB per year.
Features
The iCloud service allows users to store music, photos, applications, documents, bookmarks, reminders, notes, iBooks and contacts as well as serving as a platform for Apple based email and calendar services.
Each account has 5GB of free storage and content which is purchased from Apple can be stored without adding to the free space. With all your digital content safely in the cloud, it will be transferred to a new device by simply associating your device with the iCloud account.
Backup and Restore iOS Devices - You can now backup your iOS device online without being restricted to syncing your data through iTunes. The data can be restored back onto your device by connecting it to your computer.
Find My iPhone - This service allows you to track the location of your iPhone, iPod Touch or iPad through the www.me.com web portal. The location of the device is displayed on a map and you can display a message and/or play a sound on the device (even if it is set to silent), change the device password, or remotely lock/erase the device content. An application was released with the iOS 4.0 software update and devices running iOS 4.2.1 can download and install the application free of charge on their devices.You can download the Find My iPhone application directly on your device using the App Store of by using the link which follows -
Photo Stream - This service allows you to store the most recent 1000 photos on iCloud servers for 30 days without any charge. When a photo it taken using an iOS device it is automatically uploaded to the iCloud servers and pushed to all the other devices associated with the same Apple ID. The device will also be synced with Apple TV allowing you to view your photos on your HD TV.
iTunes Match - This service will cost $24.99 a year and is currently only available in the U.S. It allows customers to scan and match music from your iTunes music library with Apple's online store. You can download up to 25,000 songs in 256kbps DRM free AAC file format.
Back to My Mac - This service allows you to remotely login to other computers linked to the same Apple ID and have Back to My Mac enabled.
iCloud Web portal - You can manage all your information and files through the icloud.com web portal on a supported web browser. The web portal allows your to check web-mail for your @me.com address, manage your contacts, calendars and iWork files and find your iOS device using the 'Find my iPhone' service.
source - Apple