Electric strikes are often used for "buzz in" type of systems. Electric strikes come in many varieties. They can be 12 or 24 or even higher voltage and they may take AC or DC current and some even take both. They may be fail safe or fail secure.
» A fail safe electric strike needs power to keep it locked.
» A fail secure electric strike stays locked even without power.
The most common by far is a fail secure. When using a fail secure electric strike even without power it stays locked from the outside coming in. For egress or getting out, a door knob or lever on the lock allows for safe exit.