What's the difference between a fitted protector and a fully encased (zippered) one?

A fitted protector covers only the top and sides like a deep-pocket sheet and guards against spills and sweat. A fully encased, zippered encasement wraps all six sides and seals with a zipper, which is what you want for bed bug or dust mite protection. Encasements cost more and are harder to remove for washing, so most people buy them only when allergen or pest sealing is the priority.