Muggles' Guide to Harry Potter/Magic/Troll

Overview
Trolls are large, slow, humanoid creatures. They are generally destructive, and wild ones, at least, are generally not smart enough to reason with.

Extended Description
A troll made an appearance in Harry Potter and the Philosopher's Stone, when Professor Quirrell let one into the dungeons during the Hallowe'en Feast. A second one, or possibly the same one, also provided by Quirrell, was one of the obstacles guarding the approach to the Stone.

In Harry Potter and the Prisoner of Azkaban, when the Fat Lady returns to watch the Gryffindor common room entrance, a squad of security trolls is recruited to keep her safe. These are apparently not much smarter than the one that broke up the feast two years earlier, but are apparently more biddable. It is mentioned that they seem to be spending a lot of time comparing clubs, but otherwise we hear little of them. While their size is never mentioned, context would lead us to believe that they are not as large as the ones mentioned in Harry Potter and the Philosopher's Stone.

Fred Weasley has stated that all one needs to do to speak Troll is to point and grunt, earning him "an extremely nasty look" from Percy.

Analysis
It is not immediately made clear how Trolls differ from Giants, but from context we can assume that trolls are slightly smaller, have less brain, and are trainable (security trolls have been used several times, even in Hogwarts, as guards). From the fact that Harry and Ron both commented on the smell when the troll approached them in Harry Potter and the Philosopher's Stone, but that neither Harry nor Hermione noted any smell when approaching Grawp in Harry Potter and the Order of the Phoenix, would also imply that Trolls are also much smellier than Giants.

There is some small evidence for there being multiple species of troll. The "troll in the dungeons", mentioned in Harry Potter and the Philosopher's Stone, is identified there as a "wild mountain troll," is also described as being very large, and having little if any spoken language. The "security trolls" mentioned in Harry Potter and the Prisoner of Azkaban are apparently smaller, as several of them can fit in the hallway, communicate with each other, though possibly pre-verbally, and are either trained to take direction from the Fat Lady, or else determine for themselves when Sirius approaches, and prevent his entry. It is possible that the author simply thought the concept of a "security troll" would be a good thing to put into the story. At this remove, we of course can't tell whether that was the genesis of this distinct troll type, or whether she had in fact planned for there to be significant diversity in the troll race. The reader might consider whether having multiple distinct troll species explicitly defined would enhance the story.