from the movie:
Harry Potter and the Philosopher's Stone
A Troll is a magical creature of prodigious strength and immense stupidity. Trolls generally reach a height of about twelve feet and weigh up to nearly a ton. They are dangerously violent and incredibly aggressive, and then engage in unpredictable behaviour; a bit comparable with giants. Trolls originated in Scandinavia, but are now found all across Europe. Trolls feed on raw flesh, and are not that fussy about what they eat, from animals to even humans.
Troll language is nothing more than simple grunts that only Trolls seem to be able to interpret, though skilled magical multi-linguists such as Barty Crouch can understand them. Trolls understand only a limited number of Human words, and some smarter ones can be skillfully trained as security Trolls. Professor Quirrell had a gift for communicating with Trolls, and then used it to set one loose in Hogwarts School of Witchcraft and Wizardry in 1991 in an effort to steal the Philosopher's Stone. A Troll was Quirrell's contribution to guarding the Philosopher's Stone.
Artemius Lawson was an outspoken advocate for the strict restraint of trolls. He thought it was wrong to allow them to roam free, stating that "they were creatures who weighed a ton, but had brains the size of a bogey".