tomb vs sepulture

tomb and sepulture both are nouns.

Word	Noun	Adjective	Verb	Adverb
tomb	Yes	No	No	No
sepulture	Yes	No	No	No

As nouns, sepulture is a hyponym of tomb; that is, sepulture is a word with a more specific, narrower meaning than tomb:

tomb: a place for the burial of a corpse (especially beneath the ground and marked by a tombstone)
sepulture: a chamber that is used as a grave

Other hyponyms of tomb include burial chamber, sepulcher, sepulchre, mastaba, mastabah.

tomb (noun)	sepulture (noun)
a place for the burial of a corpse (especially beneath the ground and marked by a tombstone)	a chamber that is used as a grave
	the ritual placing of a corpse in a grave