is that what?和what is that?

都正确,也都可以说.不过,Is that what?用的时候比较特殊：在没有听清对方问提问时,要求对方重复所提问题：
- Is that a pagoda?
- Is that what?
- Pagoda.
- Oh,no.That's a shed.
What is that?是一个教科书里教的问句,问“那是什么?”