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.