All electrified locking devices fall into one of two categories. They either need power to lock, or they need power to unlock.  The original terms for these conditions were fail safe and fail secure.  Fail-secure means that if the power fails, the door remains secure (locked). Fail-safe means the opposite: if power fails, the door is unlocked. The confusing part of the wording is in the word “safe”.  You might not feel safe in a protected space if the doors unlocked during a power failure, but if the doors “failed safe” you would not have to worry about getting out of the building.

Failsafe locks need power applied to the lock to remain locked.  A failsafe lock opens and unlocks the door when the power is removed by an access control system or a power outage.

Fail-secure locks need power applied to the lock to remain unlocked.  A fail-secure lock locks the door when power is removed by an access control system or a power outage.

There are applications where only one type of lock is acceptable, mostly for life safety reasons, like in a stairwell of a high-rise building. There are other situations where either will work, but one type is better suited for the application than the other.  In general, fail secure locks are more common. At least if the power fails to the lock, your door is not suddenly open to the world. Batteries can be used to provide temporary power to some kinds of fail secure locks, but there are life safety codes in some municipalities that limit their use in many situations.