Who, whom和whose可用于非限制性定语从句中，代表某人或某人的，此时不能用that。如：

My sister, who is a nurse, came home for a few days.

我姐姐回家住了几天，她是个护士。

which

　　which代表物，在从句中可作主语或宾语，作宾语时可以省略，但作介词宾语且介词位于其前时不能省略。例如：

The book which helps you most are those which make you think most.

最能使你获益的书是那些最能让你深思的书。

This is a factor which we must not neglect.

这是一个我们绝不能忽略的因素。

Where is the book from which you quoted this sentence?

你引用这句话的那本书在哪儿？

　　which可用于非限制性定语从句中，代表物。如：

The book, which I bought second-hand, is made of oak.