A burial vault is an outer receptacle that houses a casket and is required by most cemeteries.  A vault protects the casket from the weight of the earth and equipment that may pass over the grave.  It also helps resist water and helps the ground above the casket from sinking in.  

Most vaults are lined and will seal after the casket is placed inside.  A concrete box, also called a grave liner or maintenance shell, is unlined and unsealed.