Note: As of iOS 8, Apple has replaced Documents in the Cloud with iCloud Drive. So if you've updated, you can check out our guide on how to set up and use iCloud Drive instead. If you're still using iOS 7 or below, continue on.
With iCloud you no longer have to fuss with traditional computer files and folders, or worry about forgetting where you put a document when you go back to look for it later. For apps that support Documents in the Cloud, all you have to remember is which app (or apps) opens the type of file you want to work on. From then on, iCloud manages everything, storing any changes you make back up to the cloud, and pushing down those updates to every device you have logged into your account. Create a Keynote presentation on your iPad at home, it's ready and waiting when on your Mac when you get to work.