Units are divided into two classes – Defensive and Offensive. Defensive Units almost useless in Offensive missions, but are extremely effective at protecting a Castle, Settlement or Beacon.
Conversely, Offensive Units are only useful when used in Offensive maneuvers, and will not be able to defend your Castle. A good tactic is to develop dedicated forces of either Offensive or Defensive Units, so that you are equally effective in both attack and defense.
Moreover, all the Units fall into one of four groups according to their Type: Infantry, Cavalry, Occult Units, or Bestiary Units. Mastering Lost Arts will unlock more powerful Units to train!