Trolls are a sad and disgusting race.  They were made by Morgouth in the 1^st age of Middle Earth to do the heavy lifting for his army of evil.

Like everything Morgouth made, they were only a mockery of ents.

They stood roughly 15 feet tall and possess massive strength. There are four basic types of trolls:  the Cave Troll, Hill Troll and Mountain Troll and Stone Troll.

The Cave Troll is common in the Misty Mountains and is often enslaved by a breed of Orcs that are called goblins by the Elves. The Tave Troll that attacked the fellowship was a prime example. They have very low intelligence, and though strong, have sluggish reflexes and simply focus on whoever is attacking.  Thus they are easy to distract.

The Hill Trolls are not a well-known species. They are most commonly seen in Angmar, Arnor, Ered Nimrais and Eregion. They are more aggressive and dangerous than Cave Trolls. Saron used Hill Trolls to push the war towers in the battle of Pelenore Fields. Their numbers severely diminished after the Return of the King. Arogorn’s patrols ether killed them all or drove them back into the mountains.

The trolls mentioned in the Hobbit were Stone Trolls.

The Stone Trolls intelligence is higher than cave or Hill Trolls. when they are exposed to sunlight they turn to stone.

The Mountain Trolls are much larger than Cave, Hill or Stone Trolls standing between 15 and 19 feet tall and unlike the other trolls are directly bred by Saron as fighting beasts.  Six of these were used to push the great battering ram, Ground. Unlike other trolls, they often wore armor.

The armor and weapons of the trolls often reveal their nature. Most trolls carried a club or hammer.  Likewise troll armor is very limited. The most common armor was a set of paladins, a helmet, and in some cases, a chest plate. 

In all, they're a disgusting, assorted, and motley bunch.  They don't make good companions.  They'll eat you as soon as look at you!  They're overall completely stupid.  Anything you hear about them being five foot tall or shorter, with purple or pink hair sticking up, is a fairy tale.