One further generalization: if it's a nymph imitation or larva imitation, it's meant to be fished under the surface film. How deep depends on how deep in the water column the fish are feeding. If it's called a dry or emerger pattern, it's meant to be fished on or in the surface film.
How to fish both at the same time?
So a dry/dropper rig would involve one dry fly tied at the end of the tippet/leader and one nymph/larva/wet fly tied to a piece of tippet which is somehow attached to the dry fly (usually a length of tippet material is tied to the bend of the dry fly hook). The term "dropper" has come to mean a fly that is intended to drop and be fished below the fishing level of the first or lead fly. A dry/dry rig would involve a dry fly tied to the end of the leader/tippet and a second dry fly tied to a length of material attached to the first dry fly (for example, you could tie a parachute Adams to the end of your leader, tie 18" of tippet to the bend of the hook of the Adams, and then tie an ant or a BWO or other small dry fly to the end of the tippet material).
Hope this helps