iCloud is an online cloud storage and cloud computing service from Apple Inc. launched on October 12, 2011. It provides means to store documents, photos, music, contacts, apps, etc. on remote servers for download to iOS, Macintosh or Windows devices, to share data to others and manage Apple devices if lost or stolen.
The service also offers an access to wirelessly back up iOS devices to iCloud, which helps a lot those who don't want to backup manually with iTunes.
To use iCloud, you must turn on iCloud with an Apple ID on a compatible computer or iOS device. Once you added or updated the data in iCloud-enabled apps on compatible devices, the data will be automatically uploaded to the user's iCloud account and then downloaded to the user's other iCloud-enabled devices.
Apple's Photo Stream (iOS 5.1 or later) and iCloud Photo Library (iOS 9.3/9 8.2/8.1/8 or later) makes it very easy to upload and share photos. With Photo Stream, photos sharing is very convenient between devices. The photos can be viewed on iPhone, iPad, iPod touch, Apple TV, Mac and Windows PCs. It stores the recent 1000 photos on your device and your iCloud account. As for iCloud photo library, the Camera Roll and My Photo Stream album are replaced with an All Photos album, in which all photos and videos are organized by date they were added. My Photo Stream will upload recent photos to share with other devices, but iCloud Photo Library stores your originals.
Calendar and contacts are synced among iCloud account and all iCloud-enabled devices. Email addresses (except for non-iCloud email accounts) like Me.com are synced across devices. As iCloud replaces MobileMe service, it offers some web-based apps to do what MobileMe did. With these apps, you can view email, calendar programs, address book through web browsers and all of these data will be up to date automatically.
The documents created and edited in compatible apps with iCloud account will be automatically uploaded to iCloud and then synced to all iCloud-enabled devices where those apps also run. Besides, you can access these documents via the web-based iCloud account as well. But on the web, you can only upload, download and delete documents instead of editing.
Music, movies and episodes purchased from iTunes Store are stored in iCloud account (only for videos from those companies that strike deals with Apple). You can re-download them to any iCloud-enabled devices.
And the iPhone/iPad compatible movies that come with DVD purchases are recognized as iTunes movies purchases and added to iCloud accounts even if you haven't them in iTunes Store.
Once enabling iCloud on your iOS device, all apps you have purchased via Apple ID. You can download and install these apps free on other compatible devices.
In conclusion, iCloud connects all Apple devices in amazing ways. It enables you to always have the latest versions of your most important things.