Trolls are loathsome creatures that have the ability to regenerate. When they are near death, they will fall face-down on the ground and start to regenerate. At this stage, the only things that can kill them are fire or acid (for regular trolls). There are a lot of fire- and acid-based spells and items to be found in the world, so killing trolls should be relatively easy.
There are many varieties of trolls found in the games: