They tend to be similar in shape to a traditional hexagonal coffin (as seen in films), are padded inside and have a hinged and lockable lid. Depending on the design they can be used so the the victim is either standing up or lying down. Most designs have a hinged flaps on the lid/door at head, breasts, and genital height to allow the top access to the sub without having to release them.
There is a very strong D/s element about the use of coffins, apart from the strong imagery associated with them. When locked inside, the submissive has very little room for movement and their body heat will quickly raise the temperature of the inside of the coffin. Because coffins do not stress or put pressure on any part of the body, they can be used for long bondage scenes providing that there is adequate ventilation.