Non lethal weapons are weapons which are not meant to kill the target. Whereas, lethal weapons are those which may or are intended to kill the target. A firearm is a lethal weapon, whereas tear gas may be considered a non lethal weapon. In most cases, the law considers some weapons non lethal, even when they can kill.
For instance, tear gas may not be meant to kill an individual, but it can happen. Perhaps the person has breathing conditions which preexisted the use of the tear gas and then they died from exposure to tear gas. In that case, a non lethal weapons became a lethal weapon. Taser guns are also considered non lethal weapons, as officers often use them to subdue subjects and yet people do sometimes die from the taser gun.
Another example of non lethal weapons are those which are sometimes used during war. In some cases,the weapons are used to instill fear and in others, they are used for tactical purposes. For example, Agent Orange was used during the Vietnam war in order to clear the fields so that the enemy could not hide. However, Agent Orange has an continues to kill people. Some veterans, as well as those that remain in Vietnam, are now faced with cancers that are known to be caused by the use of Agent Orange.
Lethal weapons are used to instill fear and gain compliance, whether or not they are actually used. The idea that the weapons could be used, is often enough to gain victim compliance or suspect compliance, because the fear of death is associated with those weapons.