This is the first image we know of which depicts a dwarf/gnome (lower left corner). Artist: Olaus Magnus (1555)
Dwarfs are a subterranean, often deformed people who live in caves and gorges. They have their own king. They are very good smiths forging strong weapon from metal. They can make themselves invisible, are often hostile to Gods and humans, but can, like trolls, do good deeds in return for humans' kindness.
First an illustration and detail from Louis Moe's great visual epic Ragnarok (1928):
Trolls in Norse Mythology
Classical representations of Ymer by N. Abildgaard, Danish painter, followed by Kai Nielsen's Ymer Well in Faaborg, Denmark
The first troll was Ymer, the oldest creature of the Norse universe, shaped by the snow and ice, he belonged to the Jotuns (same word as 'jætte' or 'giant', see Giants), who were of superhuman size and strength and always fighting the gods (Odin, Thor etc.), representing the destructive natural forces of Scandinavia - like the dragons of earthquake-ridden Japan.
One famous Norse troll is the dangerous troll Grendel in the epic poem Beowulf who embarks on a murderous campaign against a Danish settlement whose besieged king Hrothgar asks the legendary warriour Beowulf for assistance.
This is how Grendel is described at the beginning of the poem:
He was of a race of monsters
exiled from mankind by God -
He was of the race of Cain,
that man punished for
murdering his brother.
From that family comes
all evil beings-
monsters, elves, zombies.
Also the giants who
fought with God and got
repaid with the flood.
In other words the trolls and giants of the Norse period were seen as being locked in an eternal battle with humans, sometimes losing, sometimes winning, but always fighting. As in this lovely illustration by Louis Moe from his masterpiece Ragnarok from 1929: