Do you lose all your contacts, photos, music, etc.when you jailbreak your iphone?

And, is it possible to "un-jailbreak" your iPhone?

No. You don't lose any info. When jailbreaking it installs software that breaks down the restrictions. And you can "un-jailbreak" your iphone/ipod by reseting it which in all does lose your info unless you back up to a computer or icloud.