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 Peter Jackson's 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 Peter Jackson's The Lord of the Rings 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.