By specifying the handing of a lock, you tell the manufacturer how the latch is to be oriented with the door, and, which side of the lock body is rigid when in the "locked" position. If the latch nose is pointed the wrong way, then when the door is shut, instead of the latch sliding along the angled side (into the body), it will hit (on the flat side) against the frame of the door. Likewise, if the handing is specified incorrectly, you may find yourself locked in to a room instead of locking others out.
Handing is comprised of two parts: Whether the lock is a right or left hand, and whether the lock is regular or reverse.
By convention, lock handing is always determined in reference to the "outside" of the door. As shown in the diagram above, if you stand on the outside of a doorway, facing the door, the "hand" of the door is determined by which hand you would use to open the door. For example, looking at the diagram on the far left: You would use your left hand to open this door. If you used your right hand, you would have to release the knob as you entered the room, as you right arm would block your path.
Doors that open into a room are said to have regular handing, and doors that open to the outside have reverse handing. Reverse handed doors are common in public buildings, such as schools, where in an emergency you don't want to force a group of people step backward, in order for the door swing in.
