...again, this depends on your phone. They will FedEx you overnight your loaner phone, postage free, you will sync your phone, take out the sim card and place in the loaner phone and re-sync...90% of your stuff will be sync'd to your loaner phone. You will package up your phone in the same box, affix the provided FedEx label and ship it off to Apple (it goes to Texas).
They get it the next day and you can track on the Apple web site. You will see that it is going through a diagnostic evaluation. From that they will determine if it is repairable or needs to be replaced. Your replacement could be a new phone or a refurbished phone. They will FedEx it to you and you repeat the above and send them back the loaner phone.
Good Luck