Trolls appear as the main antagonists of the Norwegian horror "Troll Hunter" and are based on the Trolls of Norse legend, being much larger than humans and acting akin to semi-intelligent but aggressive animals - they are also a collective term for many connected species (just as in mythology).
The most prominent trolls in the film were the forest trolls, the cave trolls and the climatic Jotun - all of the trolls in the film are exceptionally dangerous beings with the strength and mass to kill humans with ease, though most seemed to prefer hunting animals or livestock when not exploring the remote areas of Norway.
The titular "Troll Hunter" (Hans) pursues trolls that have strayed to close to human habitats and kills them using special lights designed to mimic the sun's radiation - this causes a violent reaction in even the largest of trolls, seeming to turn them to stone (however unlike in the myths it is explained more as trolls turning into bone, due to calcium) - some trolls explode rather than petrify (such as the troll under the bridge and the cave trolls).
Another blend of science in the otherwise fantastical film was the explaination that the trolls of Norway were becoming increasingly violent and bold due to being infected with rabies.
Trolls in the movie (again like mythology) could smell the blood of Christians and went into a berkserker fury upon sensing them - this was explained by the director of the movie in the sense that Trolls see Christians as invaders who stole the reverence and worship the Norse used to have of Trolls and forced said Trolls into the wilderness as a result (trolls in mythology are also said to find the noise of bells to be painful, hence the church bells of traditional Christian churches were a source of ire for the already short-tempered beasts).
Trolls Found in The Film
The Three-Headed Giant was one of the first trolls encountered by the group and towered over an ordinary human, to the point its heads brushed against the tops of large pine trees - like most trolls it was semi-intelligent but also extremely feral (later explained as a side-effect of rabies).
The Three-Headed Giant chased after Hans and the group until Hans managed to petrify the troll using his light, which mimicked the properties of sunlight - this resulted in the Three-Headed Giant turning into a stone-like statue (in reality it was solid bone).
When asked about the nature of the Three-Headed Giant Hans explained that it was not born with multiple heads but earned them with age, he also claimed the extra heads were not true heads but rather used to intimidate other trolls and attract females.
The Three-Headed Giant is a reference to the Ettin of folklore and popular fantasy.
Unidentified Forest Troll
Troll Under The Bridge
The Jotun was the largest and most powerful troll in the movie and served as the climatic villain, it is based on the ice giants of Norse mythology and stood at roughly 200 ft in height, easily dwarving every other troll in the movie.