As far as I know, "tombs":
- Are normally associated with people who have a high status and lived a long time ago;
- They are very large and for VIPs basically.
- A tomb is a structure above ground.
Graves on the other hand:
- Are structures below ground.
That said, they must be different words with different semantic prosody, after all, people often use them interchangeably as I have seen/heard many times in various occasions! The question is that what shall I consider about them?
The question is that whether I can use them interchangeably or I have to use them in different situations? If so, which part of my provided information is reliable?