Whomping Willow is a giant magical tree on the Hogwarts grounds.
In their second year, Harry Potter and Ron Weasley accidentally crashed the Weasleys' flying car into the tree, seriously damaging it. The tree violently defended itself and nearly killed both boys. The following day, Herbology Professor Sprout had to bandage some of the tree's limbs to try and repair the damage. It also is above a secret passage to Shrieking Shack. The tree was planted specifically so the young Remus Lupin would be able to get to the Shrieking Shack to transform without hurting people.