Burning Titans are huge three-legged creatures. They behave like hammer warriors and deliver massive damage. Furthermore, standing adjacent to them causes continuous burning. When defeated, a Burning Titan will spawn a Risen Ashen Hulk of the same level.
Unlike burning from most other sources, this condition will last as long as the player stands next to the titan. So, it is best not to have too many warriors tank one of them at the same time; otherwise the monks may have trouble keeping pace with healing.
The Titan summoned by the Lich Lord at the end of the Abaddon's Mouth mission is actually a Burning Titan although it is just called "Titan". Killing it will end the mission, meaning it will not spawn another titan.