You should export your photos somewhere else, don't rely on PhotoStream as storage. Save them to an USB drive, if you prefer, or store them in online storage such as Box, Dropbox, ... A backup will save the information stored on your iPad. If you have to erase your device, or if something gets lost, it's possible to restore from a backup, and your device will show the same apps, settings, homescreen and lockscreen, contacts, ... as before. It's possible to backup to iCloud, or to iTunes on your computer. Here's more information about backup methods: https://support.apple.com/en-gb/HT203977 Hope that helps.