The Gillikin Country of Oz occupies the northern quadrant of the map of Oz. It is looked after by the Good Witch of the North, whose name is not revealed in the stories. This is not Glinda, who is the Good Witch of the South. When MGM made the movie version, they simply put the two Good Witches into the same actress. For those of you who have read Wicked, Gregrory Maguire tries to split the difference here by having Glinda come from the Gillikin Country even though she assumes power in the Quadling Country (So she's from the North, but of the South).
There was also a Wicked Witch of the North, who was named Mombi. Although she was replaced by a Good Witch, she managed to hold on to a very powerful secret. The Princess Ozma, rightful ruler of Oz, was held by Mombi, magically disguised as a boy named Tippetarius (making Ozma the only transgendered character in Oz that I'm aware of). After the enchantment was lifted and Ozma ascended to the throne of Oz, Mombi was stripped of her magic. Where she is now is anyone's guess (unless they've read The Lost King of Oz).
Other notable features of the Gillikin Country include:
The Forest of Gugu: The largest forest of the Gillikin Country is ruled over by King Gugu, an immense Leopard.
Flathead Mountain: A mountain with a large flat headland at its peak. The Flatheads live here and carry their brains in cans, since their heads are flat where everyone else has brains. They've had a long-running feud with their neighbors, the Skeezers.