What we call "trolls" are in actuality a wide variety of unrelated creatures. The term troll seems to generally be applied to creatures that are significantly larger than even a tall man, are bipedal, roughly humanoid in shape, and are aggressively predatory in nature. There is where the similarities end.
Some trolls are actual humanoids, large with stony or leathery skin. Others are far more exotic. Some trolls are fungal colonies, others are giant wormlike creatures. Others seem to have a mineral base.
The only true to commonality among trolls is that they are quite dangerous to humanoid species.