Passenger ships are primarily used for carrying passengers. The category includes ferries, yachts, ocean liners, and cruise ships. The category may also include cargo ships with adequate facilities to carry a substantial number of passengers. Cargo ships often act as freighter cruises, however they tend to be more expensive to travel in comparison to passenger ships. Passenger ships have also been commissioned as navy ships on numerous occasions to meet the requirements by naval forces. In the past, they were also used for transporting light goods such as mail and package freights.