In Norse mythology, the Jotunn were a race of frost giants with incredible strength, described as the opposition to their cousins the gods, although they frequently interbred or even intermarried with between eachother.
Their residence-fortress is known as Utgard and is located in Jötunheim, one of the nine worlds in Norse cosmology, separated from Midgard, the world of men, by high mountains and dense forests.
When they live in a world other than their own, they seem to prefer caves and dark places.
In Old Norse, they were called jǫtnar (sing. jǫtunn), risar (sing. risi), in particular bergrisar ('giants of the mountains'), or þursar (sing. þurs), in particular hrímþursar ('frost or frost giants'). They could also be called gýgjur (sing. gýgr) or íviðjur (sing. íviðja).
Jötunn (from Proto-Germanic *etunaz) probably derives from the Proto-Germanic root *etan, 'to eat', and would therefore have the meaning of 'eater' or 'man-eater'.
Following this same logic, þurs could come from the same Old English root þurst, or from German 'Durst', both meaning 'thirst' or also 'thirst for blood'.
Risi is probably a cognate of Old English rīsan, 'to raise', and would also mean 'dominant person' (similar to German Riese, Dutch reus, and Swedish rese, all meaning 'giant').
In Old English, the cognates of jötunn are eóten and eten, whence in modern English ettin and J. R. R. Tolkien's creation Ent. Ettin is a false cognate of yeti. Old English has the cognate þyrs of the same meaning.
Thurs is also the name of the rune ᚦ, which later evolved into the letter Þ.
The first living being, formed in the primeval chaos known as Ginnungagap, was a giant named Ymir.
When he slept, a giant son and a giant daughter grew from his armpit, and his two feet procreated and gave birth to a six-headed monster. Supposedly, these three beings gave birth to the race of hrímþursar (frost giants or frost giants), who populated Niflheim, the world of mist, cold and ice.
Instead, the gods claim their origin from Buri. When the giant Ymir was subsequently slain by Odin, Vili and Vé (the grandsons of Buri), his blood (water) flooded Niflheim and killed all the giants, except the one known as Bergelmir and his wife, who then repopulated their race.
The giants represent the forces of primitive chaos and indomitable, destructive nature. Their defeat at the hands of the gods represents the triumph of culture over nature, albeit at the cost of eternal vigilance.
Heimdall perpetually watches the Bifröst bridge from Asgard to Jötunheim, and Thor frequently pays a visit to the world of the giants to slay as many of their kind as he can.
As a collectivity, giants are generally attributed with a hideous appearance, with claws, fangs and deformed features, apart from a hideous size (however, not all of them are of great size, several being of the same stature as the Æsir and Vanir).
Some of them may even have multiple heads or a totally non-humanoid form; such as Jormungandr and Fenrir, two of Loki's sons, seen as giants. With bad looks comes little intelligence; the Edda more than once resemble their temperament to that of a child.
Even when named and described in more detail, they are often given opposite characteristics. Incredibly old, they carry the wisdom of other times.
They are the giants Mimer and Vafþrúðnir that Odin seeks for pro-cosmic knowledge. Many of the wives of the gods are giants.
Njord is married to Skadi, Gerd is consort to Freyr, Odin wins the love of Gunnlod, and even Thor, the great slayer of his race, loves Járnsaxa, mother of Magni.
As such they appear as lesser gods, what can be said of the sea giant Ægir, much more connected with the gods than with those of Jotunheim. None of these fear the light, and in comfort, their houses do not differ much from those of the gods.
One class of the giants were the fire giants (or muspeli), who resided in Muspelheim, the world of heat and fire, ruled by Surt ("the black one") and his queen Sinmore.
Fornjót, the incarnation of fire, was another of their kind. The main role of the fire giants in Norse mythology is to cause the final destruction of the world by setting fire to the Yggdrasil world tree at the end of Ragnarök, when the giants of Jötunheim and the forces of Niflheim will launch an attack on the gods, and kill almost all but a few.