A tomb is a structure above ground, and a grave is below ground. The body would be placed in a coffin or casket and then put into a tomb or grave.

"What is the difference between...?" The word "grave" suggests "to dig" (think of "engrave"), so graves are under the ground. Any hole in the ground can be a grave. A tomb is a structure which can be above ground or below it (the original word suggests that it's visible and above ground).