What Does It Do?
The iCloud stores certain types of data that you generate from your Apple devices. This data includes email, notes, reminders, calendars, messages, photos, music, Safari bookmarks, web pages you have open in Safari in real time, certain types of applications, and certain types of data files.
As the iCloud stores data that you have generated from a device, it also pushes that data to the other qualifying devices you have. For example, suppose you have an iPhone and a Mac. If you take a photo with your iPhone, the photo is stored on the iCloud, and then the iCloud downloads it to iPhoto on your Mac. The downloading process is referred to as pushing because it happens automatically. You don’t have to go to your Mac and tell it to download anything from the iCloud. Now suppose you loaded some photos from your camera into iPhoto on your Mac. Those photos go automatically to the iCloud and then automatically to your iPhone.