Who or That or Which or What?
"Who and its accompanying forms only refer to people.
The people who climbed that mountain are crazy.
The people whom we saw earlier looked concerned.
I saw the woman whose book won the literary award.
Which refers to animals and things, never to people.
The dog which tipped over my garbage can needs a shorter leash.
The crowd cheered as the plane which had flown around the world landed.
Please note that English grammar allows the use of whose as a replacement for which.
We encountered animals the ferocity of which was frightening.
We encountered animals whose ferocity was frightening.
Both sentences are grammatically correct, but the second is easier on the ears.
That refers to people, animals, or things.
The woman that witnessed the shooting will testify tomorrow.
The camel that carried us through the desert has died.
The explorers found the cave that hid the treasure for so many years.
What refers only to inanimate objects, never to people or animals.
I saw what happened to your wallet.
The expert mountaineer knew what he was talking about.http://www.drgrammar.org/faqs/#42