Most shooting attacks are not aimed at specific foes, but fired indiscriminately into a knot of troops. Those shots aimed more carefully are greatly feared, for no chieftain or general is safe from their vengeance.
Source: Warhammer Fantasy: 8th Edition
A model with the Sniper special rule can make a special Sniper shot instead of shooting normally. A Sniper's shot suffers an additional -1 To Hit penalty, in addition to any other modifiers, but can be aimed with great precision.
Unless making a Stand and Shoot reaction, a model making a Sniper shot can shoot at a different target from the one chosen by his unit. A hit from a Sniper shot is not distributed in the same manner as other shooting attacks. The Sniper can shoot at any model he can see, including characters or champions within a unit and so on – the controlling player simply declares which model will be the Sniper's target –cannot be taken.
A Sniper shot can even target a character riding on the back of a ridden monster or chariot if he wishes, or indeed a specific model where the target is usually rolled for randomly (such as a character who has joined a war machine).