Wicked is a musical based on the novel of the same name by Gregory Maguire. It takes place in the Land of Oz, and tells the story of many of the characters before Dorothy arrived. Elphaba, a smart, good-hearted, but misunderstood young woman will green skin, meets the pretty, popular, but snobbish Galinda at school. They become friends, and go on a journey together that takes them to the Emerald City, where they meet the somewhat corrupt Wizard of Oz, and eventually forces them to take different paths in adult life. Elphaba becomes the Wicked Witch of the West, while Galinda becomes Glinda, good witch and ally to the Wizard.
You can read Martin Denton's review of the original cast (now all gone), dated November 5, 2003, here.