With much thanks to Quaranyr in Old Vigaroe's comment's section for digging up a lot of otherwise-opaque info.

A disclaimer; I play with the Good Old Games release, not the original Steam release. There may be discrepancies between these posts and your experience as a result, if you're playing a different version; I know for a fact the GOG release has some (fairly serious) bugs exclusive to it, so this is a very real concern.

In order;

Units


Humans. Also includes an overview of various broad mechanical details pertinent to units such as how Morale works and the details of critical hit mechanics.


Dwarves.


Elves.


Orcs.


Demons.


Undead.


Neutral Animals.


Neutral Sapients.


Extra: Gremlins.

Magic


Chaos. Also includes an overview of basic Spell mechanics, such as how Intellect actually works.


Order.


Distortion.

Rage


Zerock. Also includes an overview of basic Rage mechanics, such as maximum Spirit of Rage level.


Sleem.


Lina.


Reaper.

Skills


Might. Also includes a basic overview of some broad aspects of Skills, such as Skill Rune costs. Also covers the Warrior class in particular.


Mind. Also covers the Paladin class in particular.


Magic. Also covers the Mage class in particular.

Other


Bosses. The Legend's three Bosses, which are their own category of entity that plays by its own rules. Including denying you Rage.


Companions. AKA 'wives'. I don't go into detail about children; there's other sites if you want to mess with that largely-useless part of the system.

Turn Order and Damage Types. A complete listing of the Initiative+Speed tier of every unit (Though I leave out Level, and that factors into turn order too), plus a visual representation of how units are distributed as far as damage type access goes. The short version is: they're almost all Physical.

Leadership to Health Ratios. Did you know that Dryads are the single most reliable unit type in the game at surviving punishment, as a stack? No, really. It's weird!