Other multifactor systems might include a biometric such as a fingerprint, or a small USB device you have to plug into your computer. For noncomputer systems, defense in depth is anything that prevents a single vulnerability from becoming a successful hack. It might be a deadbolt on your door in addition to the lock on the door handle, or two barbed-wire fences surrounding a military base, or a requirement that financial transactions over a certain amount must be approved by two people. A hack that overcomes one of those defenses is not likely to overcome the other as well.
