this post was relevant before iOS 8 came out. its mostly wrong now.
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iCloud backup only backs up what apple considers important. like for example
phone numbers ( contacts) , emails, notes, text messages, calendar.
off the top of my head, iTunes backs up more stuff, like for example pictures and videos that you took with the camera.
if you want for apple to save every single picture forever you have to pay for more iCloud space. and turn on iCloud photo stream
iOS 8
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i guess iCloud backs up everything, if your willing to pay for the iCloud storage