Contrary to what is represented in most modern media, there are a lot of different types of trolls in the Nordic tradition. As a Dutch person my research is necessarily second-hand, but here is a short introduction of “the common troll” as depicted by the stories I know from various countries:
Norwegian Trolls
Appearance: Very large, almost giant-like, scary and aggressive looking. Often have big noses. Sometimes have one eye, three heads or a tail.
Habitat: Usually inside mountains where they have entire palaces or hoards with rare riches, but are also found stomping through the forest. Usually live alone, but often as neighbours, where sometimes three brothers or sisters live in a row.
Characteristics: May kidnap princes or princesses to marry. Some are capable of using powerful magic. Can smell “Christian blood” or “Christian bones” and may very well eat people, especially children. Often turn to stone in sunlight.
Swedish Trolls
Appearance: Larger than humans but not huge. Often have very long, matted hair, big noses, long ears and a tail, so are often described as ugly.
Habitat: The forest or the mountains. Have a society of their own with a “troll king of the mountain”, often live in big clans.
Characteristics: Might kidnap children or youths because they like them, but could also eat them. Can leave changelings that grow up normally, but stay rather trollish at heart. Have a habit of stealing farm animals. Might be helpful or kind when treated with respect. Sometimes turn to stone in sunlight, but many are out and about during the day. Some have to keep their word or rule of trades like fae and cannot harm those pure of heart. Some possess (simple) magic.
Icelandic Trolls
Appearance: Ranging from large to huge giants, with big teeth and a terrifying, ugly appearance. Apart from that rather human, no tail for instance. Often female, while tales from other countries usually skew a little to the male side.
Habitat: Mountains and cliffs. Often live alone but as neighbours, may live in family groups.
Characteristics: Have terrifying, booming voices and are generally aggressive and violent. Fast runners and climbers, fearsome fighters. Kill and/or eat people. Are generally afraid of the sound of church bells or Christian blessings, and of the sun, which might turn them to stone.
Danish Trolls
Appearance: Human sized, often has a tail and sometimes short horns.
Habitat: The woodland, hills or mountains which might hide their treasures but may also just be big cosy camps. Live in large family groups.
Characteristics: Like to party with lots of food and drink and dancing around the fire. Sometimes steal children but never eat them. They may leave a troll changeling or a log turned into a child in their place. Some trolls wanted human children because they could go onto hallowed ground, which trolls could not. Inclined to steal things. May be befriended by leaving food and drink out for them. Has no problem going out during the day. Some can make themselves invisible to humans or use other types of magic.
The ‘trows’ from Shetland and Orkney (honorary trolls)
Appearance: Short, usually ugly or strange looking but might pass for human. Often called “the grey neighbours”.
Habitat: Inside grassy mounds and hills, called “howes” or “knowes”. Inside these mounds they have dazzling halls to have parties in.
Characteristics: Trows behave like some Swedish and Danish trolls came over to the Scottish Isles and decided to turn a bit fae. They usually only come out at night and are often invisible even then. They are fond of visiting people’s houses to play mischievous tricks on them. They might kidnap people, especially musicians or children. One story tells of them carving a wooden likeness of a wife and child to leave behind in their place.