The Scandinavian mythology creatures are separated into two groups, Mountain People (Bjergfolk) and Elf People (Ellefolk). The names are a bit deceiving though, because not all Mountain People live in the mountains, and not all Elf People are elves, but the general rule is that Mountain People live in mountain areas or have their homes underground, while Elf People live in the forest/bog/swamp/field (ghosts are their own category, because they're not a people).
As usual there are three trolls because they get bigger and meaner the father up North you get. The biggest has the lightest skin because they turn to stone during the day, so they don't get a whole lot of sun, obviously. Trolls often have magic powers, but it usually involve turning one thing into another, including living things.
Truthfully, the Ogre doesn't really belong in the picture because they're French/English/German, but they're often compared to trolls even though they're completely different species, and the trolls are a good deal older than ogres. Ogres also tend to be a lot meaner than trolls (Shrek not counting of course).
Giants and the trolls far North are often confused with each other, but giants look like ordinary but big humans, and surprisingly trolls are often bigger than giants. They can be either mean or gentle.
Unlike what Tolkien told us in Lord of The Rings, dwarfs are not a warlike people at all in Nordic mythology. They live quietly underground where they dig for metals or steal gold from burial places. They would much rather bribe their way out trouble than fight.
Finally there's the jotuns who are by far the most powerful of the peoples. They can control the wild nature and have magic powers and knowledge not even the trolls can compete with. They're incredibly strong, and back in the day people often dreamt marrying a jotun.