Apple gives every registered Apple ID 5.0GB Total Free storage on their iCloud servers to backup all your important stuff like
Messaging
Apps
Contacts
Calendar
Mail
Notes
Just to name a few, as well as your iPhone settings so you can restore these at a future date if you choose to restore a new phone or have a corrupted copy of iOS.
You can choose to purchase more space from Apple, if you like, for a yearly price listed in Settings/iCloud. Just click on "buy more storage".
This guide is to show you how to manage what gets backed up to your iCloud account to avoid paying for more storage.
It's simple. Go to Settings>iCloud>Storage&Backup>Manage Backup>This Phone>
There, you will find the size of your next Backup along with a list of what will be backed up in order of size.
The key to Managing your backups is going through your list and turning off any apps you might have deleted since your last Backup. It might be an old game you beat or some old app you don't use anymore. Simply toggle it off.
That's just a summary so if I missed something just let me know
Thank you
Sent from my iPhone 5