The journal is stored locally on your device, to ensure privacy. With iCloud enabled, the journal is also synced automatically across all of your devices.
We strongly recommend that you regularly backup all the information on your device using iTunes, or using iCloud, particularly before installing important OS or App updates. Please see this Apple support article for detailed instructions on how to back up and restore your device's content:
iOS: How to back up and restore your content (http://support.apple.com/kb/HT1766).