The Wicked Witch of the West is the main antagonist in The Wonderful Wizard of Oz, the first novel of the famous Oz series. She is Dorothy's arch-nemesis. Although the Wicked Witch only appears in the first book, and that the Nome King is the primary antagonist of the series, she was so memorable (mostly thanks to the film) that most adaptations and nearly all retellings of the story are centered about her.
While this relationship is not mentioned in Baum's books, in the movie, the Witch is the sister of the Wicked Witch of the East, who is killed when Dorothy arrives in Oz. The Witch asks aloud, "Who killed my sister?" (except with more calculation than sorrow). As a result, The Wicked Witch of the West's role is made more prominent as she seeks revenge against Dorothy for killing her sister. When Dorothy claims the death was an accident, the Witch of the West replies, "Well, my little pretty, I can cause accidents too."
It is from this movie that popular culture gets the oft-quoted phrase, "I'll get you, my pretty, and your little dog too!".
Her other motivation is to get the powerful Ruby Slippers (changed from the Silver Shoes of the book). She often, but not always, flies on a broomstick (as opposed to her umbrella in the novel), and has a crystal ball through which she can see happenings elsewhere.
Her lackeys are Nikko, the Flying Monkeys and the Winkie Guards. The Flying Monkeys kidnapped Dorothy and Toto, taking them to the Wicked Witch's castle. The Wicked Witch of the West locked Dorothy in a room of the hourglass, but Toto escaped, barking for help to the Scarecrow, Tin Man, and Cowardly Lion. The three friends disguised themselves as Winkie Guards and rescued Dorothy. But just as the four friends were about to escape, The Wicked Witch of the West sent her Winkie Guards to chase after the heroes. When Dorothy and her friends were surrounded by guards, The Wicked Witch of the West set the Scarecrow on fire. Then Dorothy noticed a bucket of water, and tossed it to extinguish the fire on Scarecrow, but also splashed some on the Wicked Witch. As a result, the Wicked Witch met her end when she melted to death. Just like the Munchkins, the Winkies cheered because the Witch was too dangerous to be left alive. Dorothy and her friends took the Witch's broom and proceeded to the Wizard's domain.