There are two good witches—one in the north, and one in the south—and one appears in each half of the story. When Dorothy first lands in Oz, she's introduced to the first (nameless) witch, an old woman who declares, "I am a good witch, and the people love me" (2.22). (She may be good, but she sure isn't modest!) Though Dorothy only interacts with her a short time, she shapes the story by sending the girl to the Emerald City and by giving her a protective kiss, which saves Dorothy's life later in the book.
The Good Witch of the South, Glinda, is "beautiful and young to the eyes" (23.3). Like her counterpart in the north, Glinda only makes a brief appearance, but it's an important one. She tells Dorothy how to get back to Kansas, for one thing, and she arranges lifts home for everyone else in the gang. Worth noting: all four witches (the good and the bad) have real magical powers, but Glinda doesn't have to use hers.