Trolls are creatures of myths and fairy tales. Usually they are described as physically monstrous giants, and they're almost always evil. The way trolls are depicted varies a bit depending on whether you're talking about ancient mythology or modern fantasy.
A Norse Legend
In ancient Icelandic myths, trolls are monsters that eat people, especially children. Some legends also depict them seducing men's wives while they are away. They usually have horrible looking faces with long hideous noses and misshapen bodies. Originally they were thought of as giants, but some legends eventually made them smaller than humans and a bit less frightening. Most legends describe them living underground, sometimes inside mounds.
The mythical trolls had an aversion to sunlight, which could often turn them into stone statues. Each trolls name was also a guarded secret because a man who knew the name could potentially use it to kill them.
Eventually the troll made its way into popular culture around the world. They've been depicted in fairy tales, and modern fantasy stories, along with film and television. Sometimes these trolls aren't very close to the original Norse mythology, but they're almost always villainous.
An example of the troll in popular culture would be the version used in the Dungeons and Dragons game universe. These are tall and skeletal with a weakness against fire.