These are the outside containers into which the casket is placed. Burial vaults are designed to protect the casket, and may be made of a variety or combination of materials including concrete, stainless steel, galvanized steel, copper, bronze, plastic or fiberglass. A graveliner is a lightweight version of a vault which simply keeps the grave surface from sinking in.
(Source: ICCFA FAQs: http://consumer.iccfa.com/faq)