Caskets are a type of container that may potentially hold treasure.
These containers come in many different forms. Some are called Sarcophagi and others Tombs.
Another variety is the Undefiled Grave. These function almost identically to above-ground Caskets, but always spawn in the same locations rather than random placement.
Tomb is another name for an Undefiled Grave, but is not the same thing as the above-ground "Tomb", or the Tombs of Tal Rasha for that matter.