The first living being formed in the primeval chaos known as Ginnungagap was a giant of monumental size, called Ymir. When the icy mists of Nifelheim met with the heat of Múspellsheimr Ymir was born out of the joining of these two extreme forces from either world in the great void. When he slept a jötunn son and a jötunn daughter grew from his armpits, and his two feet procreated and gave birth to a son, a monster with six heads.
These three beings gave rise to the race of the frost giants, who populated Niflheim. The gods instead claim their origin from a certain Búri. When the giant Ymir subsequently was slain by Odin, Vili and Vé (the grandsons of Búri), his blood (i.e. water) deluged Niflheim and killed all of the jötnar, apart from one known as Bergelmir and his spouse, who then repopulated their kind.
The Jötnar frequently interact with the Aesir, as well as the Vanir. They are usually in opposition to, or in competition with them, but also interact with them in a non-hostile manner. Some Jötnar even intermarry with the Aesir and Vanir and many are named as parents or grandparents of Aesir such as Thor and Odin.
Contrary to popular belief, the jötnar were not in fact giants, nor a different race, but a tribe of gods like the Æsir and Vanir. They were a different branch of the family and represented chaos and natural disasters. They also came in a wide variety of shapes and sizes; from human-looking, to troll-like and even being the shape of animals, and everything in between.
Some of the jötnar are attributed hideous appearances such as claws, fangs, and deformed features, apart from a generally hideous size. Some of them may even have many heads, such as Þrívaldi who had nine of them, or an overall non-humanoid shape; so were Jormungandr and Fenrir, two of the children of Loki. Yet when jötnar are named and more closely described, they are often given the opposite characteristics.
Many of the jötnar are described as beautiful, Skadi being described as the “bright bride of the gods". Although some jötnar are said to have been of considerable size, many were of no difference in size than that of the Æsir or Vanir. The Jötunn do appear to have some shared characteristics between a few of them, "according to well established skaldic precedents, any figure that lives on, in or among rocks may be assumed to be a giant". This is most likely due to their association with the creation of the earth.