Your iCloud account stores your iOS device backups as well as your synced data (contacts, calendars, notes, etc.) and iCloud email (if you have an iCloud email account). You can see how much is being used on your iPad by going to Settings>iCloud>Storage & Backup>Manage Storage.
If you want to purchase more iCloud storage, you can do this on any of your iOS devices or computers that are signed into your iCloud account, as explained here: http://support.apple.com/kb/HT4874.