A deviation is created when you run the rope through a carabiner attached to a second anchor. This pulls the rope sideways to redirect the rope away from hazards like abrasive rocks, waterfalls, etc.
In my experience, we rarely use deviations. Usually our first tool is to try to rig the drop with a single rope that goes all the way to the bottom. If there is something that prevents a straight drop, then our next preferred method is to rig it with two separate ropes. This generally requires a ledge to stand on at the lower anchor point. If that's not possible, then we go for rebelays or deviations.