iCloud Drive is a brand new feature in iOS 8 that changes iCloud from an invisible storage system into something similar to Dropbox or Google Drive. When you upgrade to iOS 8, you’re asked if you want to activate iCloud Drive. You might want to think twice before tapping the “Upgrade to iCloud Drive” button.
Before iCloud Drive, apps are using the “Documents & Data” framework for documents in iCloud. When you upgrade to iCloud Drive, documents managed under “Documents & Data” will be migrated to use iCloud Drive, and it is not backward compatible.
iCloud Drive is available only in iOS 8 and OS X Yosemite. In order for documents to sync under iCloud Drive, all your iDevices and Macs needs to be running iOS 8 and OS X Yosemite.
If you’re using only one device, if all you devices have been upgraded to iOS 8 and you do not have a Mac, it is safe to pull the trigger to upgrade to iCloud Drive. For Mac users, hold off a few weeks until the release of OS X Yosemite which is expected end October.