physics in blender game

hey,
i want to know what the ghost and invisible box is used for in the physics tab of properties window.
also would want to know what the compound check-box signifies in the collision bound section.

ghost means it doesnt collide with other objects

invisible means it cant be seen… visually and by raycast sensors etc i believe.

compound… means all collisions with children act as a collision with the parent? im not certain.