1)what = the thing which; whatever = anything
What you want has been sent here.
Whatever you want makes no difference to me.
　　
2) who= the person that　 whoever= anyone who
(错)Who breaks the law will be punished.
(错)Whoever robbed the bank is not clear.
(对)Whoever breaks the law will be punished.
(对)Who robbed the bank is not clear.
　　
3) that 和 what
当that引导定语从句时 ，通常用作关系代词，而引导名词性从句时，是个不充当任何成分的连接词。宾语从句和表语从句中的that常可省略。What只能引导名词性从　句， 用作连接代词，作从句的具体成分，且不能省略。
I think (that) you will like the stamps.
What we need is more practice.