Night latch is the correct terminology for what most people call a Yale lock. It is simply a latch operated by a turn from one side and by a key from the other side. Sometimes they are called slam locks, due to the reason that once the door is closed behind you need a key to come back through the door.
Please see details on following pages.
The majority of households have a night latch on their front door and in the vast majority of cases it is a visible surface night latch. It is possible however to have the night latch function concealed in the door as a mortice night latch. This type of attachment is common found on communal flat entrance doors.