They are generally sold in aerosol cans and are a very expensive source of pesticide. Dust formulations are made by adding the active ingredients to a fine inert powder or talc. They are generally used dry. A bait formulation is made by adding the active ingredient to an edible or attractive substance. Baits are often used to control slugs, snails, or small ground insects and rodents.
Gardeners often attempt to compare a spray with a dust.
It should be noted that dusts are a type of formulation, but sprays are not a formulation, they are one means of applying several different formulations such as wettable powders or emulsifiable concentrates that are mixed with water.