Skippered charter means that the yacht is rented with a professional crew consisting of a skipper/captain who is responsible for the maneuvering of the yacht. In several cases the skipper is aided by other crew members as well. Skippered charter is normally used for larger yachts for which a skipper/captain with documented special nautical skills and experience is required.
Crewed charter means the yacht comes with a crew. This can be anything from a 35-foot boat with a two-person team serving as captain and chef to a 300-foot boat with a squad of 30 or more crew members including stewardesses, engineers, mates, deckhands, scuba dive masters.