The lock is the mechanism inside the door that moves the bolts. When it fails, or when it is not the right one, you notice straight away: the key will not turn, it is hard to lock or the door opens far too easily.
In this group we answer the questions we get most about locks: when the whole lock has to be changed and when the cylinder is enough, what a multipoint or a hidden lock is, whether a lever lock is secure, what to fit on a shop door or an aluminium door, and what part the latch and the strike plate play.
It is useful if your lock is giving you trouble, if you want to reinforce the door or if you are choosing a new one. When you are ready, see lock installation or lock repair.