Mythology is defined as a collection of beliefs, tradition and legends of a particular society or religion. Many will tell you mythology means superstitious beliefs but is actually a faith of an individual, society or civilization which is mostly classified as "wrong" by science.
Norse mythology or Scandinavian mythology is myth of Norsemen. It is a very vast mythology like that of Greece. Many may take that this myth is same to Celtic myth of Ancient England and France, but they are very different except a few similarities in gods.
Norse mythology was creation by people of Northern Europe where it is very cold. This environmental factor has a huge effect upon the mythology of Scandinavia. They describe their gods and heroes wearing thick fur clothes unlike of Greek where they are mostly shown naked.
Even the Hell in Norse myth was described as a very cold and cruel place unlike most other myths which have it hot and fiery.
In Norse mythology before anything; soil, human, god or water existed, there was Ginnungagap, a dark abyss which separated Muspelheim; homeland of eternal fire and Niflheim; homeland of death and cruel cold. Then, it is said that, from Niflheim, came twelve streams of cold ice and from Muspelheim came a stream of fiery lava. They two met in the mid-way and formed large amount of vapor which formed a giant mass of frost. Then the frost melted later on and a giant called Ymir was born.
Then in later evening the wandering Ymir fell asleep near the land of Muspelheim which was very hot. Then, due to heat he sweated. From his sweat two giants like him were born out of his armpit.
Then from the remaining mass of the frost from where Ymir had come from, a giant cow was created. Then Ymir and his other giant children begun to feed themselves from the milk of the cow Audhumbla. Then the cow, to nourish herself, began to lick a salty ice block in front of her. From licking and thinning the ice block, she uncovered the first Aesir god, Buri. Then Buri had a son named Bor who married with one of the giantess and gave birth to Odin, Velle and Ve.
Later these three gods united with other gods thinking that giants were a danger to gods and killed the giant Ymir. The blood produced from Ymir's body was so much that it formed the seas and drowned all other giants except two who became ancestors of other frost giants.
From Ymir's skins and muscle, Odin made the soil, from his hair the plants. From his bones they made mountains and hills. From the grubs in his corpse, they made dwarves.