There are 13 Units:
Imps
For additional info: Imp
Imps are the most basic unit in the game. These are your early game meat shields or tanks.
Skeleton Soldiers
For additional info: Skeleton Soldier(SS)
Skeleton Soldiers are the real first tier meat shields. However, imps do a fine job until you can get corpse soldiers. In theory they are better than imps since they are able to level, but imps will serve fine for this tier.
Skeleton Archers
For additional info: Skeleton Archer(SA)
Skeleton Archers are your first ranged units. They are still useful in end game due to their targeting priority and low upkeep.
Corpse Soldiers
For additional info: Corpse Soldier(CS)
Corpse Soldiers are the second tier meat shields. They replace your imps (or skeleton soldiers) and will later be replaced by a Dark Knight.
Corpse Archers
For additional info: Corpse Archer(CA)
Corpse Archers are the second tier ranged and the work horse unit. They are the most used unit in the game due to their high damage and inexpensive upkeep when compared with their higher tier counterparts.
Bird Women
For additional info: Bird Woman
Bird Women are special units used to gather extra information about a battle. They are extremely weak and have a high summon and upkeep cost. They should only be summoned to get a detailed battle report for a fight, and destroyed immediately after to avoid the high upkeep.
Dark Priests and Dark Priestess'
For additional info: Dark Priest(DP) or Dark Priestess(DP)
Dark Priests and Dark Priestess' are similar units, both filling the healing niche. Dark Priestesses are the more powerful than Dark Priests, but have increased upkeep.
Dark Sorcerers and Dark Sorceresses
For additional info: Dark Sorcerer(DS) and Dark Sorceress(DS)
Dark Sorcerers and Dark Sorceresses are the third tier of Ranged. They use magic damage and thus ignore defense. They are mainly used to eliminate Knights, though if you can afford the high upkeep they can replace your Corpse Archers. Dark Sorcerers are the more powerful of the two, but have higher upkeep.
Dark Knights
For additional info: Dark Knight(DK)
Dark Knights are the final tier tanking unit and replace all lower tier melee units. They are used to distract Sorcerers/Sorceresses until your Skeleton Archers can deal with them. Note that the higher tier upgrades take Human sacrifices, the stats of the human used will be added to the Dark Knight.
Mistresses
For additional info: Mistress
Mistresses have a chance of stunning people. They will target spell casters first. Lvl 1 Mistress has a chance of stunning 1 human/creature, lvl 2 Mistress has a chance of stunning 2 people during 1 turn.
Dark Lord
For additional info: Dark Lord(DL)
Dark Lords are expensive, but well worth the cost. The Dark Lord's stat bonuses (excluding blood and defense) are added to every creature in your army.