Humans represent the "center" of the species list. Their one true advantage is a distinct lack of disadvantages. Humans are decent at most hybrid roles. They can also utilize their perfect experience rate to quickly swap roles early in the game.
Elves have a distinct focus on long-ranged combat. They quickly gain skill with Archery, Invocation, Dodging, and Stealth. Elves work well with most magical roles and some hybrid ones.
Dark Elves are twisted brethren of the more generic Elves. They quickly gain skill with Stealth, Stabbing, Pyromancy, and Sorcery. Dark Elves work well with some magical roles, some hybrids, and any stealth-oriented role.
Mountain Dwarves focus heavily on melee combat, though many are skilled smiths. They quickly gain skill with Axes, Armor, and Pyromancy. Mountain Dwarves work well with certain hybrids and any melee focused role.
Hill Dwarves are the savage cousins of the Mountain Dwarves. They quickly gain skill with Invocation and Maces. Hill Dwarves are good at most roles involving the Invocation skill. Hill Dwarves can consume corpses for a small nutritional boost.
Gnolls are brutish dog-men lacking in finesse. They quickly gain skill with Maces, Unarmed Combat, and Throwing. Gnolls work well with most melee-focused roles, but should avoid any role that places a significant emphasis on spell use. Gnolls deal extra damage on any successful melee attack thanks to their fangs. Gnolls can consume corpses for a large nutritional boost.
Harpies are a feminine species with an eye for deception. They quickly gain skill with Sorcery and Stabbing. Harpies are good at most magical roles and can also become competent stealth specialists. Harpies are naturally evasive but require more food than most species. Harpies deal extra damage on any successful melee attack thanks to their talons.
Westmen are an ancient division of humans with a tighter focus on martial combat and magic. They quickly gain skill with Swords, Pyromancy, Invocation, and Polearms. Westmen are good at numerous spell casting hybrid roles - they can also be proficient at pure magical roles.
Specters are spirits of the dead that seek adventure or a way to return to life. They quickly gain skill with Sorcery. Specters should stick to a magical role. Specters do not need to eat. Specters can cast a terrible Curse upon all nearby enemies at the cost of their ethereal bodies becoming less tethered to the physical world...
Halflings are an unusual division of humans with bizarre luck, appetites, and stature. They quickly gain skill with Throwing, Stabbing, Stealth, and Sorcery. Halflings can take up an archery role via Throwing or focus on their sneakier abilities (or some mixture of both). Halflings need to eat less than your average person. All halflings are blessed with a temporary life protection which allows them to take one lethal blow and not die.
Fair warning: this list is not necessarily "finished" nor is each species guaranteed to be in "sneak". This is merely meant to preview how different species might look in "sneak"!