Your iCloud storage was originally increased from the normal free 5GB for free until 30 September 2012 as a concession because you migrated from MobileMe. Your account storage has now been increased for free until the end of September 2013. At that time if you do nothing your account will revert to 5GB and there will be no charge. (You will find that the option to downgrade is not available because it is already in place.) If you don't at that time wish to pay to continue to use the extended storage you should ensure that your account usage does not exceed 5GB - if you exceed it at that time you won't be charged but you may lose some data. (All this has no effect on the amount of storage on iPhones, iPads or computers.) You won't at any point be charged without your specific agreement.
You only get the complimentary storage on an iCloud account which was migrated from a MobileMe account, and this has an @me.com or @mac.com login. if you did not migrate your account before 1 August 2012 it is now too late to do so.
You can purchase additional storage if you wish to, but note that this will over-ride the free storage. The additional iCloud storage levels available are: additional 10GB, $20 p.a., 20 GB, $40 p.a., 50GB $100 p.a.
More details in this Apple Help page: http://support.apple.com/kb/HT5527