what they do?和what do they do?有什么不同?请举例说明,谢谢

what they do是一个从句,它不能单独用,只能用在主句的后面,如Do you know what they do?(你知道他们是干什么的吗?)
what do they do是一个完整的句子,如果问别人What do they do?意思是问“他们是干什么的?”