The deadbolt lock works by effectively bolting the door into its frame. The bolt is “dead” in that it has to be manually moved in and out of place by means of a key or knob. A standard horizontal throw extends one inch beyond the edge of the door and into the jamb. All deadbolt locks should be made of solid steel, bronze, or brass; die-cast materials are not fashioned for great impact and could break apart.
Single Cylinder Deadbolts are the most commonly used. Single cylinder deadbolts require a key to unlock and lock the door from the outside. From the inside the door can be locked or unlocked with a simply turning the thumb turn mechanism.
Double Cylinder Deadbolts have a keyed cylinder on both sides of the door. These deadbolts require a key lock or unlock the door from both sides. Keep in mind that in an emergency you'd have to find your key before being able to unlock the door and escape.
Double or Single Cylinder?
Deadbolts come in both double and single cylinder models. A double cylinder deadbolt requires a key to operate the deadbolt from either side of the door. A single cylinder deadbolt can be locked or unlocked from inside by a thumb turn.
Conventional security wisdom has dictated that double cylinder deadbolts be used on doors with windows. This eliminates the danger of someone breaking the glass and reaching inside to unlock your door. However, there are a few reasons to re-think this approach and use a single cylinder deadbolt in most applications.
The first reason is a concern for life safety. A double cylinder deadbolt may prevent you from exiting quickly in case of emergency. You don't want to be fumbling for a key when your house is burning.
The second reason to reconsider this policy is the strength of modern windows. Back when single-pane glass was the norm, the single cylinder deadbolt was a bigger risk. The glass on most newer doors is nowhere near as flimsy.
My recommendation is that you use single cylinder, Deadbolts on all your exterior doors. If possible, use solid doors with no glass.