The Olog-hai were a fierce breed of Trolls that appeared during the last days of the War of the Ring in Sauron's service. They had none of the old Troll vulnerabilities: they were very intelligent and able to move under the sun. For this reason they were seen by some to be Orcs like the Uruk-hai, but they were definitely of Troll stock.
The Olog-hai do not actually appear in the Lord of the Rings and are only mentioned in the appendices to the book. It is however possible that some Olog-hai were present at the Battle of the Pelennor Fields.
Olog-hai are described as being taller than a man, and covered in horny scales, carrying hammers and bucklers in their claws, though there is no way of knowing whether this description applies to all of the Olog-hai, or just to those in the battle.
Olog-hai were shown as the shock troops in the motion picture The Lord of the Rings: The Return of the King, sporting armor and the famed war hammers alluded to in the appendices. These Olog-hai were not shown talking in any speech other than grunts. They differ from the other Trolls depicted in the movies in that they have more forward-facing eyes as well as hair on their bodies, suggesting that they are a more advanced form of Troll. They are usually gray to black in color. They have twenty-four teeth. Their eyes are also bright orange, showing their evil connection with Sauron.
The correct pronunciation of Olog-hai is o'log-high.