Trolls are very large (around 9 feet tall) humanoids of poor intellect. They were bred "in mockery" of Ents.
In The Hobbit they speak with thick Cockney accents. They turn to stone when exposed to sunlight and they enjoy eating Hobbits and Dwarves. While threatening, the trolls in The Hobbit serve as a comic element.
Morgoth created Trolls before the First Age. Trolls were very strong, but they turned to stone in sun light. Many Trolls died in the War of Wrath, but some survived and joined the forces of Sauron, the greatest surviving servant of Morgoth. In the Second Age and Third Age, Trolls were among Sauron's most dangerous warriors. In the Third Age, Sauron created the Olog-hai, which were more powerful than earlier breeds of Trolls. While most Trolls cannot bear exposure to sunlight without turning to stone, the Olog-hai apparently could; this attribute is mentioned in the Return of the King as making them particularly dangerous. During the War of the Ring, Sauron used Olog-hai in the Siege of Gondor and Battle of the Pelennor Fields. In the subsequent Battle at the Black Gate, the hobbit Peregrin Took killed a large Olog-hai troll. As a result of Sauron's ring being destroyed, Black Gate and the rest of Mordor collapsed to ruin during that battle. Most of the Trolls present at the battle were killed, with a very few escaping.
In The Lord of the Rings, several breeds are mentioned:
- Cave-trolls seem incapable of speech and it is not clear whether they can move in sunlight.
- Normal Trolls were strong and vicious, but stupid creatures created by Morgoth. Nobody knows how he managed to breed them though it is stated by Treebeard of the Ents that Trolls were "made in mockery of" Ents, similar to the way that Orcs were bred from captured and tormented Elves. Tolkien did not discuss in detail the ways in which "good" beings could be corrupted to evil.
- Snow Trolls as well as Hill Trolls are mentioned, but it is not known how they are related to others, and their abilities.
- The Olog-hai, unlike the original Trolls, seem incapable of speech (though it's said they know Black Speech and are able to move while exposed to direct sunlight. Apparently Sauron created them.
While in Norse mythology, the Troll was a magical creature with special skills, and are so accepted to this day in Norway, in Tolkien's writings they are portrayed as evil, with crude, bloodthirsty habits.