I'm still playing jHeretic from time to time. I've found four words about "tomb" in the map names. They are TOMB, GRAVE, SEPULCHRE and MAUSOLEUM.

According to the Chinese explanation in the dictionary,
Tomb seems to be a general word for the burial place;
Grave is the burial alcove in the ground without any "decoration" on it;
Sepulchre seems to be a religious word, probably has some abstract meaning;
Mausoleum sounds like a huge tomb with lot of "decoration" on it.

What's the difference between these words?

That's about right.

Tomb:


Grave:


I don't use the term, but Sepulcher (you had the British spelling of it) means:
sep·ul·cher    ( P )  Pronunciation Key  (spl-kr)
n.
A burial vault.
A receptacle for sacred relics, especially in an altar.  

Mausoleum:


Hope that helps a bit.

Traditionally a mid-eastern tomb carved into a rockface and sealed with a large circular stone is called a sepulchre.  This is similar to the type of tomb Jesus Christ was placed into after his crucifiction, thus explaining the religious connection.  These graves were usually painted white and contained little or no internal decoration, save for the burial "bed".