There is some debate as to how many categories are necessary in order to describe the troll race. Several notable authorities on the topic have chosen to specify a fifth category: namely, the dark trolls.
Historical capital: Zul'Aman
Known tribes: Amani, Firetree, Mossflayer, Revantusk, Smolderthorn, Vilebranch, Witherbark
Description: Due to a mutation that took place many millennia ago, the forest trolls' bodies can support plant life to a limited extent. A thin layer of moss inevitably grows on the skin of forest trolls shortly after they are born and gives them their characteristic green coloring.
Most forest trolls hold all other races in contempt and will only work with them if doing so might eliminate either a more hated foe or a greater threat. The forest trolls' hostility explains in part why they are regarded as evil by most other races. It is also well documented that forest trolls sometimes practice ritual sacrifices. Although all trolls dislike elves in general due to their contentious past, forest trolls have a special hatred for high elves, blood elves, and orcs.
History: The forest trolls ruled all of Lordaeron long before the Great Sundering. Their empire took its name from the strongest tribe of forest trolls: the Amani. In modern times, the word Amani can refer to the forest trolls' ancient empire (and thus to forest trolls as a whole) or to the Amani tribe in particular. The Amani and Gurubashi empires battled the relentless aqir until the insectoids' kingdom was split in two, but the trolls never achieved a true victory.
When the high elves first arrived in Lordaeron, they used their sorcery to drive the forest trolls out of a region in the north, where the elves subsequently established the enchanted kingdom of Quel'Thalas. The forest trolls resolved to overthrow the elven invaders and topple the spires of Quel'Thalas.
In pursuit of this vengeance, the forest trolls united under Zul'jin--the most revered forest troll in history--and allied with the Horde during the Second War. A few of these tribes joined their orcish allies on the battlefield. However, in the war's disastrous aftermath, most forest trolls were furious at the Horde's failure to carry out its promise and deserted the orcs without a moment's hesitation. Zul'jin himself disappeared after the war, but he is still greatly respected by all forest trolls, regardless of their current allegiance.
Only one tribe of forest trolls is loosely affiliated with the Horde today: the Revantusk tribe. Two other tribes of forest trolls--Firetree and Smolderthorn--are part of the Dark Horde, a renegade group that is based in Blackrock Spire and led by Rend Blackhand.