A Night latch is similar to the old 'Yale' type lock. It automatically locks on closing the door and is unlocked by a key from the outside and the handle inside. Some types can be deadlocked with the key when leaving the house, which prevents the inside handle from being released.
A High Security version conforming to BS3621 is available. This is a double security lock with an inside handle, which can be locked from the inside with a key, and automatically deadlocks on closing. These types of locks will have BS3621 stamp shown on them.