I knew something was up this morning when my wife sent me a message saying that she had received an email from Apple about exceeding 5 GB of storage and functions would stop working after September 30, 2013, if she continued to exceed that amount. There's a reason for this: former MobileMe customers (those who may have a .mac or .me email account) received a 20 GB complimentary storage upgrade when they moved to iCloud. That upgrade expires next Monday -- September 30, 2013.
To keep things running the way they have since 2011 when the migration from MobileMe to iCloud occurred, you'll need to either reduce the amount of storage you're using or buy one of Apple's storage plans. Those plans include 20 GB more space (25 GB total iCloud storage) for US$40 per year, or 50 GB additional storage (55 GB of iCloud storage) for $100.00.
Want to find out what's eating up all of that storage in the cloud? From an iOS device, go to Settings > iCloud > Storage & Backup, and then tap on Manage Storage. You'll uncover which devices are using the most storage, and you can choose to turn off certain items so that they're no longer being backed up on iCloud. This is also possible from the OS X System Preferences application -- click on iCloud and you'll see how much storage is available. There's a Manage... button that provides the same functionality as Manage Storage in iOS.
Good luck -- and take care of this immediately. If you don't, Apple will shut down iCloud Backup, Documents in the Cloud and iCloud mail until such time that you either go back below the 5 GB limit or add more storage space.