In the first of the Slayers novels, Lina Inverse describes trolls as humanoid creatures about twice the height of a man. Their skin colors vary from dark yellow to dark green, sometimes blue, and combined shades of the three. Trolls have extremely powerful physiques and seldom wear more than a loincloth.
Trolls are often used as the brute force in an army of henchmen by anyone who has the stomach to make dealings with them. They live simple, primitive lives in forests or caves and are rarely seen in areas of large human populations unless they are being used as henchmen.
The creatures are famous for their extreme regenerative properties; if they lose an arm or a leg to a sword chop, they simply regrow the lost limb within the next several seconds. There are only two ways to certainly kill a troll: behead it, or destroy it utterly with magic. The latter is the most famous example; when Lina and Gourry are set upon by Zolf with a squadron of trolls, Lina enchants them with a reversed Recovery spell. When Gourry gives them a tiny wound, they lose their constitutions and disintegrate. Simply using magic itself to slay a troll does not guarantee its actual death; the half-troll Dilgear was shot full of holes by a Flare Arrow spell from Zolf, and, though he did not appear again in the animated series, was confirmed by the author to have regenerated his wounds and come back to life. Only a spell that results in complete incineration or disintegration, or possibly freezing the creature solid or petrifying it and then shattering it, will successfully kill a troll.
In the episodes of the first season of Slayers during which Ruby Eye Shabranigdu is resurrected from Rezo, large numbers of trolls brazenly begin sacking villages and 'celebrating' the return of the ma-ō. This seems to indicate that they are in some way aligned with the mazoku. It is even said that their kind first appeared in the Kōma War, during the first rise of Shabranigdu. Like a number of other "lesser monsters" such as minotaurs, orcs and berserkers, trolls appear to be purely physical creatures that are nonetheless spawned by the influence of Ruby-Eye.
Because they live entirely on the material plane, unlike mazoku, it is likely that trolls reproduce like other creatures of flesh and blood, such as humans - in the anime's first season, there is an instance of a troll immediately abandoning its attack on Lina Inverse to go chasing after a pretty human girl, which implies that trolls may even be able to crossbreed with other species. This is given further possibility by the existence of Dilgear, a lupine beastman/troll hybrid, and is explicitly assumed in the non-canonical Slayers roleplaying game Slayers D20. Even if trolls cannot crossbreed, they are still a popular target for wizards dabbling in the arts of chimera construction, due the inherent strength, toughness and, most of all, regenerative capabilities of trolls - this would explain Dilgear's hybrid nature if mundane crossbreeding is impossible, as he was an employee of Rezo, a skilled chimera maker.