I think, just a personal key which is nothing to wallet but is needed to spend your Bitcoins associated with a Bitcoin Wallet. Generally, offline wallets are considered as a secure wallet because it can not access even when there is no online connection. If you are using offline wireless it is your responsibility to take the necessary security alerts to keep you physically safe. I do not really believe in web wallets or exchanges because there is always someone who is trying to hack web wallets and exchange databases. This article can help you understand the different types of your Bitcoins preservation